Only once in my life have I been involved in a project that made a test user feel physically sick.
We started out with good intentions. One of the tedious parts of a travel search form is dealing with the calendar popup - from date, to date, click click click. Couldn't it be nicer and faster? Plus we knew that Brent was continually irritated when other sites had faster calendar popups.
So Sam led the effort to create a circular menu where in a single gesture you could sweep from your departure date to your return date smoothly and wonderfully. In order to gain the fine control of choosing exact days at either end, but let you sweep quickly inbetween, we implemented a fish-eye effect, so the area around your mouse pointer was expanded.
The upshot was a menu that moved under you in a disturbing way, the circle creeping round as you moved your mouse. The user in question moved the mouse pointer resolutely to the opposite side of the screen and refused to touch the control any longer, as it made her feel queasy.
We always preferred a clear reaction to indifference.