Check whether your masks actually have the same number of points and the same settings for the anchor points. This behavior is typical if they don't match.
I'm with Mylenium on this. In 90% of the cases I have seen you cannot add a point to the mask. Try deleting the keyframes and then animate the vertices again being careful not to add any or get them out of order. Remember that by default vertices move in a straight line from one keyframe to another.
Do you guys mean that the masks themselves should have the same number of points?
I'm a little confused by this, whether it's the source and destination masks or something to do with the correspondence points -- it seems like it would be a little difficult, though not impossible, to make the source and destination masks have the same number of points.
I tried changing the anchor points both within the pre-comps and in the main comp to match, but that didn't solve the problem.
And as for the keyframes, the problem shows up even when there are no keyframes (as it stands there are only two), as when I just drag the percent in the Reshape effect it does the same weird thing.
One more question: When I pre-comp one of the images it says the anchor point is some odd point (which shows up as different from the coordinates suggested by the info panel in the top right of the screen, which show pretty close to what it looks like, 640, 360. Any idea why this anchor point might be showing up weird.
Thanks so much for all the help.