Have never had a problem with sickness when a full motion sim is 'doing its thing', with all the rocking and rolling going on. However, have had instances of the 'leans' when the motion is off, but the visuals are still moving - that can play havoc with your brain and inner ear.
Old simulators that may have a bit of free play in the motion have also given me a few instances of the leans - the motion gear hits a notch in it's geometry but the visuals continue to move. Wibble!