The other day I tightened the slipper just to the point of getting wheelies - before that you could hear it slip, and while no wheelies it still ran well. But while out this time the slipper loosened completely to the point the truck would just crawl.
I took it apart and tightened for the rest of the battery pack, but this has never happened before. Is it a sign the slipper needs to be rebuilt?
Found out why, the slipper bearing is blown:
Untitled by ishbits, on Flickr