dovecot - Jan 2005 - 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

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