I know this is a feature, and on occasion as pointed out a useful one. But in some cases it can be a tad annoying as the OP probably considered it.

I had a recent example where a lingering .pyc made my test suite pass (and me nearly push broken code) despite the fact that I'd moved the file away and edited it, with bugs. Obviously I now remember to rebuild and remove all .pyc's before I commit, but it took a near miss to remind me to.

Would it be useful to have a command line flag that did something like this (perhaps that test runners could set by default) to prevent others from running into similar situations?