OS - Windows 10 64 bit; CPU - Third-gen i7; GPU - ATI HD 7xxx; RAM - 24 gigs
Broken: Blender hash 3b5dec4 (March 22 build from the buildbot).
Short description of error
The display percentage parameter (in the particle panels) breaks after rendering (so if you set it to show only 1 percent of the amount of particles in your hair particle system, exiting out of the render window will show 100 percent of the particles instead (which means a major slowdown for really large systems)). Clicking the parameter field again and clicking out of it will lower the visible count back to 1 percent.
I will also note that this bug only affects hair particles (when you have the particle mode set to "emitter" the parameter works like it should at all times).
Exact steps for others to reproduce the error
Open the .blend file and notice that only a few hair particles are visible, then render the scene. When the scene is done rendering, exit back to the 3D view and notice that every particle is visible despite what the display percentage suggests.
This has annoyed me for a while at least, I hope it can be fixed.