Quote Originally Posted by Buffalobiian
"Can you guess what I'm thinking of?" is similar, since "of" is tacked on by people who thought "Can you guess what I'm thinking?" didn't sound "right" to them.
It is incorrect to end a sentence with a preposition. However, in this case, they didn't simply tack it on. For example:

"Can you guess what I'm talking about?"

About is a preposition, and cannot be omitted in this case. Still, the format is technically incorrect. It has only become accepted because it is used so often.

"Can you guess what I'm talking?" is simply wrong.