508 Loop Detected
The server walked into an infinite loop and had the honesty to say so.
Another inheritance from RFC 5842: with bindings, a folder can contain itself, and a Depth: infinity traversal would descend forever. 508 aborts the operation and confesses: found a cycle, not going to finish.
It is the deep cousin of 208 — 208 avoids repeating what was already listed; 508 gives up when the repetition is inescapable. Outside WebDAV, some proxies borrow it to report internal redirect loops.
HTTP/1.1 508 Loop Detected
Content-Type: text/plain
Depth: infinity traversal found a bind loop