error in pop.log

hi,

this is the first time i see this error - i've been using qpopper for 
many years on
lots of different machines. it only has come up one single time:

  -ERR [SYS/TEMP] maillock error 'Max tries exceeded' (4) on 
'/var/mail/xxx': File exists (17)

what does this mean?
why the "file exists" ???
same mailbox was polled 2 minutes later without an error... :-|

thanks

MK