A bad Roll Auto Tune on a edge case sub250 quad produced this:
ATC_ACCEL_R_MAX,Infinity
Mission planner upon the next connection loaded some parameters via MAVFtp, then a few the old way and stopped resulting in no parameters being displayed. It took me awhile to figure out that is was the Infinity value because after putting a value in there using QGC the parameters loaded normally.
This really should not happen, the parameters should load anyway even with a none numeric value which Auto Tune produced.