I got through ~10 quests without any issues until this happened:

The item being requested, Experiment Beanbag, was removed from my inventory. I refreshed and got the "you don't have all the items" error. So I went back, bought ANOTHER beanbag and got the same error a second time when trying to finish the quest.

Chrome, Win 7

[edit]

Looks like a few of us posted a board at the same time, can you please lock mine? Thanks and sorry for the hassle!