On one of my recent cruises my lovely wife, who is actually nice enough to pack for me, made a small mistake and forgot to pack my underwear.
Now - I had a pair on, and for some reason she did pack one special pair of wash & wear undies - made from a material that dries within a few hours.
So, between the two pairs I was able to make it though the cruise - although it was a challenge.
On other cruises I have forgotten: black socks (I only had white ones, not good for formal nights), a necktie, cufflinks and a bowtie for my tux, and more...
WHAT IS THE WORST THING YOU HAVE EVER FORGOTTEN TO PACK?