One of those very-hard-to-track-down, trivial-to-fix kind of problems:
without this patch, TCP roundtrip time measurements will corrupt the
routing cache's RTT estimates under heavy network load (the bug causes
RTAX_RTT to go negative, but since its type is u32, you end up with a
huge positive value...).  From there on, later TCP connections quickly
will go south.

The typo was introduced 8 months ago in v1.29 of the file by the patch
entitled "Cleanup DST metrics and abstrct MSS/PMTU further".