Occasionaly, we see ethernet issues from CAT5 etc., and a RB switch (450,493 etc) port gets switched to 10mbps. We have script that runs each hour that emails us if it happens.
Many of them also have script scheduled that also will turn auto-negotiate off and back on, and usually the port resyncs back at 100mbps.
We just upgraded a number of them to 5.11 from 4.17.
Now, if you turn off auto-negotiate, then back on, it always goes to 'failed'. The only way to get it back to 'done' is to reboot. We've downgraded a couple of RB450G's this morning, and it goes back to working.