I always forget something when I go on holiday. I think most people do.
Sometimes it's the thing you've left right next to the front door to make sure you don't forget it. Often you're so busy trying to remember the obscure things that it's the obvious things you forget. Sometimes the things that get left behind are the things you remember a few days before travel and you think "I must remember that" and it's almost instantly dismissed to the deepest recesses of the mind.
This time I forgot binoculars. This might not seem an essential item (I agree) but I'd seen a guidebook recommendation that it was a good thing to pack if you have children; it allows them to look at detail that they might otherwise miss. This particular guidebook had challenges for children that required binoculars.
I forgot a bikini. This isn't strictly true as I don't own a bikini. I threw all of mine in the bin during a moment of self-loathing in which I decided my midriff should never be seen in public. Having seen other stomachs on display here I think I could have got away with one. Also, we have a private garden here that isn't overlooked so the only people's eyes I'd be offending would be family. So this is a retrospective "with hindsight" omission. And I could still debate the merits of a bikini. I think there comes a point when it simply isn't right.
And I forgot knickers. This hasn't been the disaster one might imagine. I had packed some for our epic journey and I'd also packed laundry detergent. This and a quick trip to local shops has negated the need to go commando.