Head-of-Line Blocking
One delayed or lost item stalling everything queued behind it, even when the later items are ready.
It occurs at several layers and the distinction matters when choosing a protocol.
At TCP, a lost packet stalls delivery of everything after it on that connection, because TCP guarantees ordered delivery. HTTP/2 multiplexes many streams over one TCP connection, so a single lost packet stalls all of them — which is why HTTP/2 can be slower than HTTP/1.1 on lossy networks.
At HTTP/1.1, a slow response blocks the connection, which is why browsers open six per origin.
HTTP/3 over QUIC solves the TCP-level case by running on UDP with per-stream ordering, so loss affects only the stream that lost a packet.
It also appears in message queues (one poison message blocking a partition) and in thread pools.