Getting the “please raise pressure foot and turn off machine” message. 

The short answer is "all of them", because the message just means that something failed during the power-on self-test. But in reality the only one you can actually clean is the one under the needle plate in the back right corner. It's got 3 tiny switches that trigger depending on what plate is attached, and a bit of lint can trick it into detecting the wrong plate. Other than that, it kind of depends on what model machine we're talking about. Different models have different causes for this ever-so-annoying message.