I forgot my bathing suit once... but didn't run out and buy a replacement since bathing suits are expensive. It was a 3-day cruise and it rained the whole time anyway!
On the same cruise, the night before the cruise I realized I forgot my cardigan sweater I wear with everything (I am cold-natured.) We were staying at our departure port and had our own car, so luckily we were able to dash over to the local mall the morning of the cruise and I found a replacement at JCPenney. I was so glad, too - the air-conditioning was a little overenthusiastic on that ship!