I've got a handful...old and recent, but the one that stands out the most right now was August 16, 2012. We were at WDW for my older daughter's surprise bday trip and I got so caught up in certain details that I forgot to put my video camera in my bag and forgot to charge the battery for my DSLR camera.
We had a 7am breakfast at Chef Mickey's and I'd ordered a custom cake to be delivered during the meal. It wasn't until just before they brought the cake out did I realize that the video camera was back in the room. Thankfully, my DSLR still had a charge at this point (not that I was aware of the battery for that either) so I snapped a TON of pictures, BUT it would have been nice to have the whole thing on video since Minnie showed up at the table to "sing" happy birthday with us just as our server lit the last candle. DD5 was soooooooo surprised and it was the PERFECT bday cake moment.
We didn't have time to run back to the room to grab the video camera, so I figured we'd just rely on snapshots with the DSLR. That all worked well until about 10:15a when my DSLR battery died.
In this time, I missed pictures of the Sunshine Seasons staff going wild over the bday girl (serenaded her with a variety of instruments), both girls in Bruce's head over by Nemo, DD5 gushing about her first "flight" on Soarin', DD5 finally getting to ride Maelstrom, DD3 meeting Mulan while she slept, and lunch at Le Cellier.
DH had suggested I buy a disposable camera, and I guess maybe I should have done it, but it's so hard to limit yourself to 24 photos when I often take hundreds per day.
So, I regret not having and properly charging my cameras on probably the biggest day of our most recent WDW trip.