strange error requiring restart of postmaster

Hi!

Here's a strange thing that just happened:

register=# vacuum verbose analyze personer;
NOTICE: --Relation personer--
NOTICE: Pages 144: Changed 0, reaped 10, Empty 0, New 0; Tup
5451: Vac 25, Keep/VTL 0/0, Crash 0, UnUsed 36, MinLen 92,
MaxLen 332; Re-using: Free/Avail. Space 14644/1188;
EndEmpty/Avail. Pages 1/7. CPU 0.00s/0.40u sec.
NOTICE: Index personer_personid_fname_idx: Pages 32; Tuples
5451: Deleted 0. CPU 0.00s/0.01u sec.
NOTICE: Index personer_fnamn_idx: Pages 27; Tuples 5451:
Deleted 0. CPU 0.00s/0.01u sec.
NOTICE: Index personer_personid_index: Pages 21; Tuples 5451:
Deleted 0. CPU 0.00s/0.01u sec.
NOTICE: Rel personer: Pages: 144 --> 143; Tuple(s) moved: 0.
CPU 0.00s/0.00u sec.
NOTICE: FlushRelationBuffers(personer, 143): block 142 is
referenced (private 0, global 1)
FATAL 1: VACUUM (vc_repair_frag): FlushRelationBuffers
returned -2
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset:
Succeeded.
register=# \q

(normal vacuum gave same result)

I tried restarting postmaster, and surprisingly, it helped!
what is this?

Cheers,
Palle