I have had two occasions in life where I was sitting in a public lunchroom and shot my mouth off with a critical opinion of someone/something, only to then realize that a person in the group I was criticizing was sitting right next to me or right across from me. Once in college, once in high school. Still feel bad about both of those two.
Another major foot-in-mouth moment I think I've told on here before: I worked at a bank briefly several years ago, and we had to do these things called "medallion signature guarantee" (think like a notary on steroids -- have to provide multiple forms of ID, have a banking relationship, etc.) Anyhow, a married couple comes in and needs one of these signature guarantees. I should have asked them up front what the purpose of their visit was, but I instead asked them to get out their multiple forms of ID first. The man made a lighthearted comment about having to provide all this documentation, and I jokingly said "Yeah, they pretty much make you sign away your firstborn child!"
About 2 minutes later, I ask what they needed the signature guarantee was for, and they explained that their firstborn child had died, and they needed this paperwork to transfer her college savings plan to another beneficiary. I felt like the world's biggest jerk.