After rendering, particle combing data is lost. The conditions for this to happen are:

1) particle modifier must be disabled in the viewport

2) this only happens after the render

3) apparently, the model must be set to a multires level different than the one set for rendering.

After you render you will see that the combing data is corrupted randomly (It may happen to all the particle systems or just to some of them).

How to fix this:

_ With the particle systems enabled in the viewport, change the preview level of the multires modifier to match the level that is set for rendering. Then, the combing data will be restored at all levels.

_In some cases, going to multires level 0 or disabling and enabling the multires modifier, also fixes the problem.

This example file was created with rev 32657

http://dl.dropbox.com/u/3834396/particles_multires_bug2.blend