I opened the Minecraft Launcher, and I logged in and went to the menu, then I logged onto my server and the first thing I saw, was that the menu bar at the top of my screen was black and flashing. Then I noticed that in minecraft, the terrain rendering was flashing black really fast, and then the entities were flashing too. I also noticed that signs were acting really buggy and it would flash from a normal sign to a sign with random white characters on it.

EDIT: I finally managed to get the problem again, i posted the triggered crash log and screenshots.

P.S. The fake players that it shows that where on the server at the time are citizens from the NPC bukkit plugin, but i posted the bug here because it seemed like it was more vanilla minecraft related, and i'm also using no client mods.