Broken: 2.77a (official build) and 7ac126e (from builder.blender.org)
Short description of error
Particle strands combing gets corrupted after an F12 render
Exact steps for others to reproduce the error
Press Shift Z and let render preview. That is the correct result
Stop preview and press F12 for a tiled render. Shows incorrect result
Esc and press Shift Z again for incorrect result. Combing has now been corrupted
EDIT: This just got weirder. This is a new particle system created from scratch. It has never been combed!
Press Shift Z and let render preview. Notice the correct length of the strands.
Stop preview and press F12 for a tiled render.
Esc and press Shift Z again. Now strands are much shorter in the preview!