[Dovecot] Errors: file istream.c: line 135... assertion failed

I'm getting thease messages in log file:

file istream.c: line 135 (i_stream_skip): assertion failed: (stream->v_size == 0 || stream->v_offset + count <= stream->v_size) kernel: pid 23364 (pop3), uid 1042: exited on signal 6

It doesn't happen very often, but I'd like to know what is causing them, any ideas? Should I be worried about it ? System is freebsd, format is mbox and dovecot is 0.99.12.1

-- PB - burcharp@iem.pw.edu.pl