Let's assume, without revoking any of today's science, that the world is a simulation.

What would a bug look like?

I'm assuming that "the eiffel tower suddenly being bent at 45°" is rather unlikely, the same way you don't see a bunch of clowns appearing in the middle of a game of need for speed. So what is likely?

* repetitions / deja-vus?
* changes in gravity / speed of light?
* ...