A bathing suit believe it or not. More than once. I now have quite the collection.
My wife forgot a bunch of stuff including her suits when we went down in September. This was a day after the hurricane, we were sure we wouldn't be able to get in so we very slowly packed some stuff. Late Sunday morning we got the call the house was good to go. Two hours later we were off...... forgetting to pack a bunch of stuff.
A few years ago we went on a 14 day cruise. I thought I was smart and packed my carry on and suitcase about 2 weeks before. Stupid me forgot underwear, luckily I not only brought a few pairs of bathing trunks I also brought for whatever reason 3 pairs of Speed'O's.....which substitute very well Lol