Yes, there is an 8th night in FNaF2 ! After you have completed Night 5, if you quickly keep scrolling over the "Continue Game" button it will show Night 8 instead of Night 5. It is a bug or an easter egg, or just an unfinished feature? Who knows... But if you can finish this hard night, the "6AM" win screen will glitch and stay forever. I think an admin should add a Night 8 page.
If you still not believe me, try it out yourself!