The question is in the title. This could include a glitch by the game with ragdolls or maybe having a friend make you laugh at something that you just considered formidable etc. Basically anything that made you remember you were sitting down on a couch.
For me it was the
If you have been after the first time they remove you from the Animus to hit the switches it isn't a spoiler.
For me it was the
time you went into a trippy Altair sequence. While climbing the tower at the end I bumped Alair's head into a lamp on a string. And for the whole time I was climbing the tower and even after and during cutscene on top of the tower the lamp was flying around on an orbit around the pole it was tethered to. A shame too, for I finally had started to accept the animus load screens, which meant they no longer seemed to be screaming LOADING at me while I waited.