Here's a longstanding bug in Gnus, still present in Oort 0.02:

I have a group nnml:cvs with associated spool file
~/incoming/cvs.spool.  I also have a spool file called
~/incoming/freebsd-ports-cvs.spool with no associated group.

Getting new mail for nnml:cvs also slurps the freebsd-ports-cvs.spool
file into a non-existent nnml group because they share a suffix.

Bad juju.  Here's a fix.