#1 on my list has got to be the myriad of LaTeX-writers who think that math variables can be longer than a single letter. That is, they write things like:

#1 on my list has got to be the myriad of LaTeX-writers who think that math variables can be longer than a single letter. That is, they write things like: $foo + bar$ ... not realizing that LaTeX is typesetting these to imply f times o times o + b times a times r. It looks horrible. A simple fix would be: $\text{\it foo} + \text{\it bar}$