I woke up and this is what I found in the log file:

4100:20081202:072243 Query::insert into history (clock,itemid,value) values (1228231363,4363,54368664179395.906250)
4100:20081202:072243 Query failed:PGRES_FATAL_ERROR:ERROR: numeric field overflowDETAIL: A field with precision 16, scale 4 must round to an absolute value less than 10^12.

4100:20081202:072243 Query::select num,value_min,value_avg,value_max from trends where itemid=4363 and clock=1228230000
4100:20081202:072243 Query failed:PGRES_FATAL_ERROR:ERROR: current transaction is aborted, commands ignored until end of tr
ansaction block

4041:20081202:072243 One child process died. Exiting ...

It's rather disconcerting to see my monitoring application die. It seems that there was not sufficient sanitization of data before it was inserted into the "history" table.