There are many anecdotes about Paul (Pál) Erdős. Here is one of my own. At a combinatorics conference in 1978 at Colorado State University, participants stayed in a dormitory. The bathrooms were shared by two dorm rooms. Each bathroom had two doors, one to each dorm room. I shared my bathroom with Erdős. The first night around 3:00 a.m. I was awoken by a loud knocking on my wall. Erdős couldn't get into the bathroom! It turned out than when I last used it, I locked the door to his room and forgot to unlock it when I was finished. I got up and unlocked the door. Needless to say I was very embarrassed and profusely apologetic. The next night I was extra careful not to repeat my mistake. Nevertheless, I was again awoken in the middle of the night by a loud knocking on the wall. I couldn't believe it! I had checked several times that I did not lock Erdős's door. How could I have done it again? When I got up to unlock the door, I found that it was already unlocked! Erdős in fact had locked the door on his side and had failed to notice this! Now it was his turn to be extremely apologetic. In fact he wrote me a note of apology.