Light source.
I have now seen two "paragon" level parties go on an extended, multi-session dungeon-crawl without thinking about bringing a light source. They bought magical potions... spare ammo... scrolls... everything except a humble torch.
How ludicrous did it get? Well...
About five years ago, the first party (3.5e, 17th level) teleported into a hostile dungeon. Once inside, they realized it was pitch black. Nobody had any way of creating light... except the wizard. Who eventually had to use the only spell he had available to create light: limited wish. That's right... he burned 300 XP for want of a length of flaming timber. The PCs were rightly appalled at their oversight, and it has become a legend at our table ever since.
About two months ago, they did it again (4e, 14th level). Only two of the four players were from the "same group", but they all knew the story. They descended a long shaft into the Underdark, and then realized nobody had bought any sunrods. They ended up losing a healing surge each as they spent hours crawling through tunnels looking for some vegetable matter (giant mushroom stalks) that they could light up using an at-will Fire power.