I am usually very aware of my surroundings, and according to SO and friends, am perhaps too concerned with the feelings of others around me, particularly strangers.
A few days ago, I was eating at a restaurant with a close friend and we were discussing his weight. We both played a lot of sports growing up, exercise a lot, and are pretty think-skinned when it comes to making fun of each other about pretty much anything.
Friend leads off by asking how he currently compares to his fattest self (note: friend is probably like 15 lbs beyond being very athletic, and looks to be in very good shape). We proceed to joke for 25 minutes about how huge he was when he was a bit heavier a few years back...what his SO will do when she returns to the country in a few weeks and sees him in his current state. Friend mimics Vince Vaughn from True Detective saying, "Sometimes...your fattest self...is your best self. Know what I mean?" Discuss how he was winded after a recent 5-mile run (he was a competitive runner).
This convo goes on for way too long, and to anyone else listening we probably sound like vain assholes. "How fat do you think you could get if you did A,B,C by X date if you tried your hardest to get huge?" Deep, deep thought experiments. We then get up after paying our bill, and I turnaround to see that seated right behind me, presumably the whole time, are two obese people. Not sure how much they heard, but I felt like absolutely horrible. I never want to offend anyone, and I am sure these two heard us joking non-stop about each other's descent into middle-aged failed athletes.
I think that's the rudest thing I've ever done in a long time. I just got up and quickly left - hoping they some how didn't hear it all. Asked friend how he could let us talk about that for so long without telling me who was right behind. He didn't seem to care - thought they didn't hear us and was still focused on trying to estimate how fat he could get by X date if he tried his hardest.
That was pretty bad. Guessing it won't rank high here, though.