Term Kind Topic What it is
Head-of-Line Blocking concept HTTP/1.1, HTTP/2 & HTTP/3 One delayed item stalling everything queued behind it, even when the rest could have been processed - the failure mode that ordering guarantees create.
Idle Connection Race Keep-Alive Race, Idle Pool Reset concept HTTP/1.1, HTTP/2 & HTTP/3 The unavoidable window where a server closes a pooled connection just as a client sends a request on it, producing a small error floor that is worst when traffic is lightest.