Along the lines of your question, Bart Ehrmann, who reads classical Greek and Latin fluently, comments about the margin notes in various early manuscripts of individual New Testament books, along the lines of "Do not alter this, it is important" and "I had to alter this because they used the wrong word," etc. This accounts for the 85,000 word discrepancy in a compilation that comprises 110,000 words.

As far as Roman graffiti goes, the best preserved examples are probably the one from Pompeii, for obvious reasons. But there are also examples from Hadrian's Wall and visitors to Egypt basically already wrote "I was here" on all kinds of monuments in Roman times.

I can't tell you anything about non-Roman acient graffiti, unfortunately (except that I'm very sure that it existed)

Maeshowe is a large Stone-age burial chamber in the Orkney Iskands (off the north coast of Scotland) from 5000 years ago. When antiquarians broke into the chamber in the 19th century they found they weren't the first such visitors. The walls were covered in runic graffiti from the 12th-century -- the islands having long been under Viking/Norse control. You can read some of them here. Some highlights are:

• "Tholfir Kolbeinsson carved these runes high up"
• "These runes were carved by the man most skilled in runes in the western ocean"
• "Ingigerth is the most beautiful of all women" (carved beside a rough drawing of a slavering dog)
• "Thorni f*cked. Helgi carved"

https://www.orkneyjar.com/history/maeshowe/maeshrunes.htm

Here's some, not ancient but still neat.

There is a graffiti on the Palazzo Vecchio in Florence. There's a legend going there that says that it was made by Michelangelo (there's actually a drawing he did in the Louvre's collections that looks a lot like it). Not exactly funny though. Still, imagine a graffiti being drawn by Michelangelo !

I heard about a rune inscription from the Varangian guard in the Hagia Sophia that could either be a name or the word “bored” 😂