since this thread is restarting in debriefing mode: one thing struck me as a non-committer following python-dev.
It seems that we (non-committers) have a difficulty making the distinction between pre-implementation design discussions (PEPs beeing the typical example), where relevant technical comments are welcome, even on minor points, and post-commit discussions, where the maintainer already made the design decisions, and discussing anything except real bugs is unhelpful.
In this particular thread, the distinction was all the more blurred by the very generic title. A thread titled something like "Commit-xxx: should mock try to detect user typos?" would have been much less likely to attract comments discussing principles in the abstract, without taking the context into account.