We have only had one or two instances of this, but it is WEIRD!
Print out a BOm Availability for an asembly.  You get the parts for the assembly, and parts from other assemblies show up.  Almost impossible to make it happen repeatably.  Very Strange.
 
Also, is there any database cleanup that we should do every now and then?  Reindex, etc?  I know this is has been mentioned in other threads, but I curious if someone came up with a recommended maintenance routine for the database.