I've written a lot of lisp, including a respectable number of macros, and I don't think I've ever been caught out by hygiene.
I've definitely written broken macros that expanded to the wrong thing though.
miniblog.
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What's the nicest way of doing clientside form validation without annoying the user?
I keep finding forms that show red errors when I write 'wilfred@' for an email or '0' for a telephone number because I haven't finished inputting the value.
LLVM's lit tool for testing files is the most successful testing DSL I've ever seen. It's used a decent number of projects outside of LLVM.
By contrast I don't see many folks using e.g. Cucumber these days.
One big challenge of open source is that the number of users (and bug reports) is entirely independent of the number of maintainers. Both users and maintainer capacity can fluctuate wildly.