Any particular thing that you need pretty soon.
--A particular book you want (unless it's by John Grisham)
--Something your kid needs for school
--men's pants (good luck finding the right waist and inseam....plus style)
--jeans (plentiful but there are probably 300 pairs to go through)
--really high quality cookware (yet I've found Le Creuset and even 2 pieces of Allclad)
--outerwear when it's cold (easy in summer)
--Christmas sweaters around Christmas (easy in summer)
--valuable stuff (yet every now and then...I've never found anything valuable)
--travel gear, especially for men
--good shoes that one can wear (lots of high heels donated, however)
Yet you will find the hard-to-find stuff (not when you are looking) when you are there at the thrift anyway, looking for the things that are easy to find.
And even if you don't see an AllClad pan, the money you save on potholders will enable you to afford the desired object.