I have two, actually.
The more noticeable is Obsessive-Compulsive Disorder.
Seemingly inconsequential things cause me a lot of anxiety if I don't do them in a certain way. For example:
When I walk on a tile floor with a checker pattern, I compulsively walk in such a way as to be sure my toes are always on squares that are in some diagonal pattern (basically, like the usable squares on a checker board).
I compulsively chew in patterns ... usually either a 1-to-1 or 2-to-1 pattern.
When I go from one room to another, I also require a 1-to-1 pattern of steps, so that I take the same number of steps in the room with each foot.
When I ascend or descend stairs, I compulsively do so by either skipping no steps the whole way or skipping steps in the same pattern the whole way.
It needs to be noted that these behaviors, in and of themselves, do not lend evidence to OCD. It's the fact that they are done out of compulsion, and that a lack of doing so causes a noticeable level of anxiety. If I haven't done one of these things, I'll default to compensating the next time. I'll stutter-step leaving a room, or chew an extra time or two, or barely tip a step with my foot on the way down a flight of stairs. I don't do it out of volition, as it is completely behavioral. I was always picked on as a grade school student for freaking out if I took more steps with one foot than another in a room. It sucked.
However, my case is not nearly as severe as some.
The other is, I actually have a mild tic disorder, which I know a lot of people associate with
this kind of behavior and Tourettes.
Most people who have a tic disorder, however, exhibit just a small characteristic constantly like sniffing, throat clearing, coughing, or twitching. It almost never results in random vocal outbursts (though I've been told that that is a possibility). For me, it's a constant clearing of the throat or minor coughing. I get a lot of "God bless you," "Would you like something to drink?" and "Do you need a lozenge?" I've gotten to the point where I just say thank you, and if they offer something, I accept it. It's just easier than explaining it.