@wez @sebastian I rather think of it as "do not mock what you don't control", i.e. if you cannot be sure you see all the API and behaviour changes in the code, then don't mock it, because at some point the assumptions underlying your mock will break. And you probably won't notice until it's too late.

The quote resonated with me especially well since TYPO3 and its community have a history of using far too excessive mocking.