Think too much.
Many years ago (I was about 20), long before I was religious, there was this young woman. We were already friends, on our way to becoming very good friends, and we also had many mutual friends. She was probably the most beautiful woman I have ever known even to today, she was also one of the nicest women I've ever known even to today, and she was highly intelligent. We had similar interests and world views. She was basically the perfect woman. While we were friends, the longer I knew her and the more time I spent with her the more I realized that I was developing some strong feelings for her and wanted to be much more than friends.
So, I asked her out. However, I asked her out normally (i.e. "would you like to go to dinner and a movie") and didn't specifically mention the word date (i.e. "would you like to go on a date"). Well, she said yes and we went out. However, since I wasn't crystal clear in my intentions it kept bugging me that I didn't know if it was a date or two friends going out. I figured she could have taken it either way (yes I was stupid, especially when I refused to read her signals that she was interested). We went out a few times but this kept bugging me. So instead of being smart (I was too young, immature, and timid back then) and having a heart to heart to let her know my intentions I wimped out and simply stopped seeing her except with our group of friends.
Fast forward about 2 or 3 years (she moved about a year before this and we had totally lost touch). One of her best friends was dating a friend of mine. Somehow this woman came up, I mentioned my interest and explained the situation that I wasn't quite sure if she took it as dating or two friends going out. Well, the friend had a "aha" look on her face and explained that this woman really was taken with me, knew I was very interested in her, and was confused about why we never worked out. After her friend confirmed my stupidity, about a year after it was too late to try to rectify the situation (the woman involved and I had lost touch), it probably took me a year or two to completely get over it (I was rather pissed at myself).
So would that count as "dumbest thing done on a date"?
If not, I have one other really bad thing I did on a date.
I wasn't always religious. Back around when I was 19 or 20 I met a young woman at a friend's college (I can't remember if it was right before or right after the above situation). We went to a party, it was our first time out, we were both extremely drunk. I got drunk fairly often back then, but I was more drunk than usual, we were alone, things were getting romantic, then I threw up on her. Ended things pretty quickly there.