If you take a "real" English threaded bottom bracket assembly and try to thread it into an American bottom bracket it will fit but the threads will be deformed and the tops of the threads cut off - it will actually have the threads reformed to American threads. This actually has no effects save that because you're reforming the threads it will screw in with much more force than you would expect.

This is why I'm trying to make people understand this - People who are not mechanics at heart and try to combine the two may not understand that they do not screw together as like threads would. And because the tops of the threads are being sheered off the English threads you may have to back off and clean the threads of detritus. Luckily "English threads" means American threads these days and you don't have these sorts of troubles save on older steel bikes with unsealed bottom brackets. But they do turn up now and again.