A bug is an error or mistake in code. This may be caused by missing semicolons, faulty logic, being bad at writing code, not sacrificing enough to the gods, and many many many other ways. Timchen is the undisputed god of them. A bug is one of the programmer's worst fear and may sometimes be called "features".
You can report sightings of these in Repl.it in the Repl.it Bugs Page. If you have any ask people to solve them on the Repl.it Talk page.