For example, I played in a Star Wars RPG, where the DM told me that I couldn't try to pilot a stationary X-wing in a hangar. He didn't ask for a check, even though my character had a pilot skill. That would be an example of "Mother May I" in my opinion. He could have asked me to make a skill check (not "Mother May I"), or he could have just set the DC impossibly high (soft "Mother May I", but almost just as bad). What he did not do, is allow me to just take the action and/or have a fair chance at pulling it off.