I vaguely remember shooting myself in my foot by doing some weird stuff with deep attrset transformations, where I managed to screw-up those "special" attrsets by making a mistake in a function that tried to build module options dynamically from another "transplant" module's options. Surely, that was weird stuff, and sadly I don't have a history of what exactly I did back then (it was couple years ago), but I remember having to introspect with Nix REPL to understand where I messed up.

I would've loved it more if Nix had ability to create custom types, ideally GADTs. Tagged attrsets work for sure, but a better type system would've probably made things easier and nicer.