At work we are getting a 64-bit version of our software up and running at the moment. Most of the usual culprits reared their head - assuming that a pointer and integer had the same size etc etc.
One more interesting one, which I've not come across before is related to using STL
string::find and the special constant
string::npos. This is not unique to our code base when you google for it and actually just boils down to data being truncated before a comparison. The nuances of the problem do lead on to a discussion about signed vs. unsigned integral types in C++ and the handling of comparisons between differently sized data types. I though it was worth looking at a bit further and definitely something to watch out for when doing code reviews.
It could also make for a particularly challenging interview question 😉