Head-of-line blocking
Beginner
When one stuck item at the front of a line holds up everything behind it, even items going somewhere else.
Novice
A packet that can’t move blocks the packets queued behind it, even if those are headed to an idle destination. Pausing a whole traffic class on a link, as PFC does, causes it.
Expert
Victim flows that share a queue or a paused priority with a congested flow lose throughput. PFC works per priority, not per flow, so a single hot spot can slow unrelated traffic several hops away.