Too many open files in system FATAL2

got the following happened yesterday:

postmaster: StreamConnection: accept: Too many open files in system
postmaster: StreamConnection: accept: Too many open files in system
postmaster: StreamConnection: accept: Too many open files in system
2001-08-30 03:04:27 FATAL 2: InitOpen(logfile 3 seg 199) failed: Too many
open files in system
Server process (pid 21508) exited with status 512 at Thu Aug 30 03:04:27
2001
Terminating any active server processes...

then like 20 mess like that:

2001-08-30 03:04:27 NOTICE: Message from PostgreSQL backend:
The Postmaster has informed me that some other backend died
abnormally and possibly corrupted shared memory.
I have rolled back the current transaction and am going to
terminate your database system connection and exit.
Please reconnect to the database system and repeat your query.

and DB restarted well after

Any idea why this happen and if i can do something to fix that ?

also i have huge amount of:
2001-08-30 01:26:43 NOTICE: BEGIN: already a transaction in progress
2001-08-30 01:26:44 NOTICE: COMMIT: no transaction in progress

and i have no idea why i have those notice too

thx in advance for help