Here's a scenario for you: A billiard ball is hit into the side pocket of a pool table, which has been conveniently replaced with a wormhole. The other end of the wormhole, thanks to a relativistic time dilation, is slightly in the past. Both ends are now at rest with respect to each other. Assuming conditions are perfect in terms of the shot being lined up, the ball comes out of the wormhole earlier than it entered the other end, hitting itself and knocking itself into the wormhole, sending it into the past. Lather, rinse, repeat. Would the ball only go through once, and be perceived as happening over and over by an observer? Or would it go through infinitely many times (until it crumbled and fell apart) but only seem to go through once to an observer? I'll put my answer behind an lj-cut.
I think the ball would continue through the wormholes indefinitely, aging until it fell apart. Observers would see it hit itself only once, but each transit through the wormhole would create its own timeline, like the Sawtooth pattern described on the Time Travel Movies site. The loop/sawtooth is finally broken when the ball has aged so much that it falls apart upon impact with itself. Or is it? Because then the ball never went through the wormhole, disrupting causality. The many-worlds interpretation doesn't have this problem, though, since the causality is maintained; the ball just came from another timeline (whereas the guy running this site believes there is only ever one timeline, though that line is discontinuous).
I'm so glad someone decided to Ask MetaFilter about time travel.