The assertion happened because of an asymmetry between the control points
for recording the start and end of a young collection by CMS, which
led to (when ScavengeALot was used) mismatched calls for switching
an interval timer on and off (or more specifically for a call to switch
off an interval timer that had not been switched on).

The fix is simple and low-risk, and consists of not recording
young gen statistics in case the collection is a result of a full
mark-sweep compaction request.

It would therefore be a good idea to include this simple low-risk
fix in Tiger.