I noticed that net.if.out (and also net.if.in) returns random values from time to time.

Host: test
Status: PROBLEM - Severity: Warning
Trigger: Outgoing errors detected on interface eth0
Event: 2012.02.07 20:14:43 - Age: 0m
Last value: 6921

after a few minutes
Host: test
Status: OK - Severity: Warning
Trigger: Outgoing errors detected on interface eth0
Event: 2012.02.07 20:24:44 - Age: 0m
Last value: 0

If I go on the machine and run an ifconfig, the TX errors counter for interface eth0 is indeed 0.