I'm not a really organized person, and I tend to be nervous before traveling (I've come to accept that this is normal, though), so I'm really prone to forgetting stuffs. Not important stuffs like passport or ticket or hotel reservation. Also not Maliq or camera or reading materials, although I once did forget to bring a novel. It's more like 'small' stuffs (which are not always small) that you just realize you forget to bring when you need them. So I think I just make a list of them here.
Flip-flops. These are especially when I'm wearing shoes, but not really necessary when I'm wearing sandals which I like to do as much as the weather and, well, the occasion permit.
Shower cap. For... shower, of course. Not always necessary in Indonesia, though :D
Chargers. Cellphone, camera and Maliq's. I once forgot to bring the camera charger and the battery did run out :(
Compass. Well, I can always do the direction guessing by the sun position, but compass was invented for reasons. And while we're at it, it's useful to know the qibla direction and the local praying time before going and not afterward.
Sunglasses. Not too keen on these, but yeah, they're useful.
Pashmina. My multifunction gear, from windshield to emergency praying mat.
Padlock. And the combination key :P
Plastic bag(s). For just in case. I usually bring my multifunction shopping bag as well.
Post-It notes. Just love them. I write important (and unimportant) stuffs on them and if I happen to have a guide book with me, I can easily attach them to it.
Reading lamp. Awkay this is a new stuff. It's important if I stay in a hostel that doesn't give a personal bed lamp, and even more important if I end up in a lower bunk. I just bought mine on my last trip (and feeling so stupid because I couldn't remember any of my Spanish while the shopkeeper didn't speak English). It's a little bit expensive (or maybe just normal price) but sooo much better compared to the cheap boekenfestijn one.
Plastic bag clippers. Especially in Indonesia where everywhere is so humid.
(The list will grow, I guess)