After all these years, I still run into Full/Half duplex mismatch issues. I can’t tell you how many ways I’ve seen people create this scenario.
I’m also surprised how many times this has been an issue from day one, but not noticed or reported until weeks or months later giving the impression the, “Problem came from nowhere”.
One of the things I do out of habit is look for errors on switch, router or anything with SNMP to enure that things are ‘normal’.
In this video, I cover various ways of performing this check.