Raised by the HPACK decoder when the cumulative decoded header-list size of a single header block exceeds the limit advertised via SETTINGS_MAX_HEADER_LIST_SIZE (RFC 7540 §6.5.2) and computed per RFC 7541 §4.1 (`name.Length + value.Length + 32` for every entry). This is the primary defence against the "HPACK bomb" amplification attack where a handful of bytes on the wire (e.g. repeated indexed-header references into a populated dynamic table) expand to many megabytes of allocated header strings on the server. The HTTP/2 layer catches this and tears the connection down with GOAWAY(ENHANCE_YOUR_CALM); a stream-level reset is not safe because the dynamic table may already be in a partially-updated state by the time the limit trips. The carried `int` is the decoded size at the point the limit tripped.