7.4RC2 PANIC: insufficient room in FSM

I was a bit stunned last night when I found this in the server logs for a
7.4RC2 installation:

Nov 24 20:37:18 x pg_autovacuum: [2003-11-24 08:37:18 PM] Performing:
VACUUM ANALYZE "clients"."x"
Nov 24 20:37:19 x postgres: [13904] PANIC:  insufficient room in FSM
Nov 24 20:37:19 x postgres: STATEMENT:  VACUUM ANALYZE "clients"."x"

Following this is of course the fallout of backends shutting down and PG
recycling itself with no other problems. Did I miss something along the
way about the FSM needing to be sufficiently large to hold all free pages
no matter what?

I plan to bump up the FSM size anyhow (perhaps tonight I can get some FSM
stats from manually vacuuming), but my gosh, that's some bad behavior for
a presumably minor situation. IMO, that's a significant bug.