When I write code that has multiple, interrelated parts, before I ever run it I step through it in the debugger to verify that my logic is correct, I didn't make any off-by-one errors, library calls return what I expect, etc.

I especially do this when the code has destructive side-effects (a file gets moved/deleted, a database get updated, whatever), so I can skip over any external actions I don't actually want to occur until I'm ready for a full test run.

Everybody does this, right? Right???