Although it is true that God appeared in visions to people (Genesis 15:1; Genesis 28:13; 1 Kings 3:5;9:2;11:9;22:19-22; Isaiah 6:1-5; Ezekiel 1:26-28; Daniel 7:9-14; Amos 7:7; Revelation 5:1-8), some argue that is the only way God ever appeared to anyone because Jesus said "no man hath seen God at any time".

People who saw God

Then the LORD appeared to Abram and said, “To your offspring I will give this land.” So he built there an altar to the LORD, who had appeared to him. -- Genesis 12:7 (ESV)

It doesn't say that Abraham was dreaming or seeing a vision.

When Abram was ninety-nine years old the LORD appeared to Abram and said to him, “I am God Almighty; walk before me, and be blameless, that I may make my covenant between me and you, and may multiply you greatly.” Then Abram fell on his face. And God said to him, "Behold, my covenant is with you, and you shall be the father of a multitude of nations." -- Genesis 17:1-4

Genesis 18 describe the appearance of YHVH like a "man":

And the LORD (YHVH) appeared to him by the oaks of Mamre, as he sat at the door of his tent in the heat of the day. -- Genesis 18:1 (ESV)

We continue reading that YHVH appeared like a man to Abraham. However, some suggest that this "man" could also have been "the angel of the LORD" who often appears like a man, but speak and acts on behalf of God.

Then Moses and Aaron, Nadab, and Abihu, and seventy of the elders of Israel went up, and they saw the God of Israel. There was under his feet as it were a pavement of sapphire stone, like the very heaven for clearness. And he did not lay his hand on the chief men of the people of Israel; they beheld God, and ate and drank.

The LORD said to Moses, “Come up to me on the mountain and wait there, that I may give you the tablets of stone, with the law and the commandment, which I have written for their instruction.”

-- Exodus 24:9-12 (ESV)

This cannot be a single man's dream because there were 74 witnesses.

Stephen was certainly awake when the people was busy stoning him:

And he said, “Behold, I see the heavens opened, and the Son of Man standing at the right hand of God.” -- Acts 7:49 (ESV)

Yet no one ever seen God

Jesus said:

No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him. -- John 1:18 (KJV)

So what did Jesus meant with "No man hath seen God at any time" if we have proof that many people did see God with their eyes?

Paul also wrote:

I urge you in the sight of God who gives life to all things, and before Christ Jesus who witnessed the good confession before Pontius Pilate, that you keep this commandment without spot, blameless until our Lord Jesus Christ’s appearing, which He will manifest in His own time, He Who is the blessed and only Potentate, the King of kings and Lord of lords, who alone has immortality, dwelling in unapproachable Light, Whom no man has seen or can see, to Whom be honor and everlasting power. -- 1 Timothy 6:13-16 (NKJV)

This cannot be a reference to Jesus, because many people saw Jesus.

There are 2 possible explanations:

To "see" means "to know"

Strong's Concordance:

In many languages, “to see” is a common idiom for “to know.” In the Hebrew language, one of the definitions for “see” (Hebrew = ra’ ah) is “see, so as to learn, to know.” Similarly, the Greek word translated “see” in verse 18 (horao) can be “to see with the eyes” or “to see with the mind, to perceive, know.” Even in English, one of the definitions for “see” is “to know or understand.” For example, when two people are discussing something, one might say to the other, “I see what you mean.”

The usage of “see” as it pertains to knowing is found in many places in the New Testament. Jesus said to Philip, “Anyone who has seen me has seen the Father” (John 14:9). Here again the word “see” is used to indicate knowing. Anyone who knew Christ (not just those who “saw” him) would know the Father. In fact, Christ had made that plain two verses earlier when he said to Philip, “If you really knew me you would know my Father as well” (John 14:7)

-- Spirit & Truth Fellowship International

In other words, Jesus meant no man had "known" God as they knew Jesus.

For the law was given by Moses, but grace and truth came by Jesus Christ.
No man hath seen God at any time;
the only begotten Son, which is in the bosom of the Father, he hath declared Him.

-- John 1:17-18 (KJV)

In other words, some know about God only by the law (teachings) of Moses. By those teachings alone, "No man hath seen God". However, by "grace and truth" which come by Jesus Christ, who "declare" (represent or model) who the Father is, the disciples got to know God.

People would die if they see God in His full glory

The LORD said to Moses... "you cannot see my face, for man shall not see me and live." -- Exodus 33:20 (ESV)

This was an answer to a request of Moses that asked that God would be present with Israel:

And he (Moses) said to Him (God), “If your presence will not go with me, do not bring us up from here. For how shall it be known that I have found favor in your sight, I and your people? Is it not in your going with us, so that we are distinct, I and your people, from every other people on the face of the earth?” And the LORD said to Moses, “This very thing that you have spoken I will do, for you have found favor in my sight, and I know you by name.” Moses said, “Please show me your glory.” -- Exodus 33:15-18 (ESV)

And God warned Moses that no human can survive the full glory of God.

This could explain why God appeared like a "human" to people when he communicated with them and Jesus possibly meant no one has really seen God in His full glory has he did.