The file containing a sample scene is in the Sample Files section with a title
of bendexpr.zip.
This file shows a problem involving bend and a float expression controller
for the bend angle.
1. Create a cylinder with enough length segments to bend it.
2. Apply a bend modifier.
3. Make sure the animation has 100 frames (just so the numbers I give
later are correct)
4. In the track view, assign a float expression controller to the bend
angle. The expression is 30*sin(360*NT).
5. Frames 50 and 100 will have major problems when the bend angle is
at -0.00 and 0.00. Frame 0 is fine with a bend angle of 0.00.
6. Changing the bend angle expression to 30*sin(360*NT)+.01 fixes the
problem in this case (no frames have bend angle of 0.00).
I've experienced the same problem with other objects.
J.J.