[Ndymium]

Having done this same mistake myself, it's just such an easy mistake to make. Most likely you're writing two or three programming languages in the same project, and the others work differently from Python. So it's very easy to miss an errant `foo={}` in a function definition when reviewing a commit of several hundred lines. Especially as the only indicator of something being wrong is the lowest priority warning PyCharm has, meaning the background colour `{}` is only slightly different.

These days I'd set up a linter that enforces these things do not happen, but it's easy to be smart after the fact. You say "once you've been trained by bitter experience it's very easy to spot", which I feel is true. You just need that bitter experience first, and for me it came in the form of a production issue.