Polo shirts with khakis and gym shoes- this is a great look if you work at Best Buy, CompUSA, or Target, but otherwise don't do it. I used to work at a place where this was the uniform for trade shows, and I tell you, I felt like an N-E-R-D everytime I suited up. Yes, I like computers, no, I don't want to look like I work on them for a living.
Jeans with no belt- Just stop. Really.
I'll let you guys come up with some more.