Going to Bahamas, I forgot contact solution. That was a hassle because the sundries shop at the hotel didn't have any. There was also no local stores nearby. I think the hotel had to call another store for it. That was one expensive bottle of contact solution.
During past trip to WDW, I forgot contact solution AGAIN. But this one was easier because it's Florida! They had it everywhere so we just stopped by Publix to pick it up.
The one thing I couldn't pick up at Publix was ... a bra. Yes. I forgot to pack bras. I had packed sports bras for the park and was actually wearing a sports bra (more comfortable for the 2-day drive). It ended up not mattering much though. As long as I had some support!
And I'm typically not forgetful! I normally makes lists ahead of time, pack everything the morning of since I still use some of the items the same day (ie. I don't have a separate vacation toothbrush), and never leave anything important behind. This past vacation was stressful because I had to bring work with me so that ended up being my main concern. Packed all the books and teacher study guides and textbooks and laptop ... all of that, I had!