I usually forget to bring the note sheets. But only the hardest pieces, which I was practising at home and hence forgot them on the stand.
Sometimes I forget the strap of course. This doesn't trouble me on double bass gigs. On these, I tend to forget the bow; but this is more intentional than not and clearly proves my high humanity standards.
Oh yeah, I almost forgot: I was often forgetting to bring a stomp box or two, which I solved by pedalboard, and later, much more satisfactory, with not using more than one pedal at a time.
I underdstand that compared to trumpet players forgetting their mouthpieces, or drummers forgetting their sticks, my problems are really mediocre.