Win 8.1 x64, 3x gtx580
Short description of error
Motion blur shutter length not keyable.
Could be called a "bug" only assuming it was "blanket" locked with the rest of parameters for no specific reason. Otherwise it's a miss of freezing start/end frames for sequenced animations as well as selective artistic exaggeration.
Exact steps for others to reproduce the error
Insert a keyframe to Motion Blur Shutter