Probably not the worst thing I've ever done, but it still kinda haunts me. A couple of years ago I was on the bus and this couple got on. I had headphones in and was reading, so everything seemed normal until about thirty minutes later. I heard what sounded like someone clapping a mosquito and looked around, everything seemed mostly normal so I returned to my book. A couple minutes later I heard it again and realized the guy was hitting his girlfriend on the thigh. He did it a couple more times and I just froze. I didn't know if I should say or do anything and I didn't. They got off the bus later and I've felt like shit about it ever since. To this day I constantly push myself to do the right thing and find out the best ways to react to scenarios like that in case they come up. I hope that girl is okay...