Although it would be unscriptural for any man to say that he has seen God as He truly is, there are, however, many different examples from the Old Testament in which men have seen various manifestations of God (akin to His "back parts"), here upon the earth. Here are some examples of the various forms in which God has manifest Himself.
EXODUS 3:2 And the angel of the LORD appeared unto him in a flame of fire out of the midst of a bush: and he looked, and, behold, the bush burned with fire, and the bush was not consumed.
EXODUS 3:3 And Moses said, I will now turn aside, and see this great sight, why the bush is not burnt.
EXODUS 3:4 And when the LORD saw that he turned aside to see, God called unto him out of the midst of the bush, and said, Moses, Moses. And he said, Here am I.
EXODUS 3:5 And he said, Draw not nigh hither: put off thy shoes from off thy feet, for the place whereon thou standest is holy ground.
EXODUS 3:6 Moreover he said, I am the God of thy father, the God of Abraham, the God of Isaac, and the God of Jacob. And Moses hid his face; for he was afraid to look upon God.
This burning bush was a manifestation of God here on the earth. This is not God in His true form (for no man may see Him, and live), but it is a manifestation of God. Moses knew that he stood before God: Moses hid his face for [Moses] was afraid to look upon God. (See also section 4.5.1).
This happened when the children of Israel departed from Egypt in the days of Moses.
EXODUS 13:21 And the LORD went before them by day in a pillar of a cloud, to lead them the way; and by night in a pillar of fire, to give them light; to go by day and night:
EXODUS 13:22 He took not away the pillar of the cloud by day, nor the pillar of fire by night, from before the people.
It was the LORD Himself that went before them—as a pillar of a cloud (by day). This is not God in His true form; this is a manifestation of God here on the earth.
This happened each night in the days of Moses, when the children of Israel had left Egypt.
EXODUS 13:21 And the LORD went before them by day in a pillar of a cloud, to lead them the way; and by night in a pillar of fire, to give them light; to go by day and night:
EXODUS 13:22 He took not away the pillar of the cloud by day, nor the pillar of fire by night, from before the people.
It was the LORD Himself that went before them as a pillar of fire (by night). Neither the pillar of a cloud by day, nor the pillar of fire by night is God in His true form. These are two different manifestations of God here on the earth. It should be noted that these two do not imply "one God in two persons."
This took place at Mount Sinai, when the Ten Commandments were given:
EXODUS 19:16 And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled.
EXODUS 19:17 And Moses brought forth the people out of the camp to meet with God; and they stood at the nether part of the mount.
EXODUS 19:18 And mount Sinai was altogether on a smoke, because the LORD descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.
EXODUS 19:19 And when the voice of the trumpet sounded long, and waxed louder and louder, Moses spake, and God answered him by a voice.
EXODUS 20:18 ¶ And all the people saw the thunderings, and the lightnings, and the noise of the trumpet, and the mountain smoking: and when the people saw it, they removed, and stood afar off.
EXODUS 20:19 And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die.
EXODUS 20:21 And the people stood afar off, and Moses drew near unto the thick darkness where God was.
EXODUS 20:22 ¶ And the LORD said unto Moses, Thus thou shalt say unto the children of Israel, Ye have seen that I have talked with you from heaven.
And, at a later scripture:
EXODUS 24:15 And Moses went up into the mount, and a cloud covered the mount.
EXODUS 24:16 And the glory of the LORD abode upon mount Sinai, and the cloud covered it six days: and the seventh day he called unto Moses out of the midst of the cloud.
EXODUS 24:17 And the sight of the glory of the LORD was like devouring fire on the top of the mount in the eyes of the children of Israel.
Although they did fear for their lives (Exodus 20:19), had the children of Israel actually seen God in His true form, they all would have died there on the spot (Exodus 33:20, in hindsight). In His true form, God is not smoke, nor is He thunderings, lightnings nor a devouring fire, yet God said (at Exodus 20:22), "Ye have seen that I have talked with you." This frightening sight, with all of the thunderings, the lightnings, the devouring fire and the smoke, was a manifestation of God, upon the top of Mount Sinai. They were God's "back parts," so to speak (cf. Exodus 33:20,23). This was a manifestation of God (see Deuteronomy 4:10-19).
EXODUS 24:9 ¶ Then went up Moses, and Aaron, Nadab, and Abihu, and seventy of the elders of Israel:
EXODUS 24:10 And they saw the God of Israel: and there was under his feet as it were a paved work of a sapphire stone, and as it were the body of heaven in his clearness.
EXODUS 24:11 And upon the nobles of the children of Israel he laid not his hand: also they saw God, and did eat and drink.
(This resembles what John described at Revelation 4:2-6, though it's not necessarily the same.) Had they seen "the face of God", that is to say, had they seen God as He truly is, then they would have died, as God later said, "there shall no man see me, and live" (Exouds 33:20). However, just as Moses would see God's "back parts" (Exodus 33:23), so too have these men seen the glory, or, a manifestation of God. What these men saw was not God as He truly is, but was a manifestation of God. It must be understood in this way, for Moses (who was there, and "saw the God of Israel," at Exouds 24:9) later said:
DEUTERONOMY 4:11 And ye came near and stood under the mountain; and the mountain burned with fire unto the midst of heaven, with darkness, clouds, and thick darkness.
DEUTERONOMY 4:12 And the LORD spake unto you out of the midst of the fire: ye heard the voice of the words, but saw no similitude; only ye heard a voice.
DEUTERONOMY 4:15 Take ye therefore good heed unto yourselves; for ye saw no manner of similitude on the day that the LORD spake unto you in Horeb out of the midst of the fire:
Likewise, Jesus Christ would later make the general statement:
JOHN 5:37 And the Father himself, which hath sent me, hath borne witness of me. Ye have neither heard his voice at any time, nor seen his shape.
JOB 38:1 Then the LORD answered Job out of the whirlwind, and said,
Again, this whirlwind is not God as he actually is, rather, God was manifest upon the earth in the form of a whirlwind.
GENESIS 3:8 And they heard the voice of the LORD God walking in the garden in the cool of the day: and Adam and his wife hid themselves from the presence of the LORD God amongst the trees of the garden.
GENESIS 15:1 After these things the word of the LORD came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward.
GENESIS 15:5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.
GENESIS 15:6 And he believed in the LORD; and he counted it to him for righteousness.
GENESIS 15:7 And he said unto him, I am the LORD that brought thee out of Ur of the Chaldees, to give thee this land to inherit it.
GENESIS 15:8 And he said, Lord GOD, whereby shall I know that I shall inherit it?
GENESIS 15:9 And he said unto him, Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon.
GENESIS 15:10 And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not.
GENESIS 15:17 And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces.