MTU mismatch
When two connected interfaces have different configured MTUs (maximum packet size). A notoriously sneaky cause of failures: small packets go through fine, but large ones are dropped silently — giving symptoms like "the internet works but some sites hang" or OSPF neighbors that get stuck.
Symptoms: ping (small packets) works, so the link seems OK, but large packets (a file transfer, a TLS handshake, an OSPF database exchange) disappear → hard to troubleshoot since everything "almost" works. Classic: OSPF neighbors get stuck in ExStart/Exchange because they can't exchange their (large) database packets when the MTU differs — OSPF requires matching MTU to form a full adjacency. Other cases: VPNs/tunnels that add overhead and shrink the effective MTU, or a misconfigured jumbo-frame link where only one end has a large MTU. Diagnosis: ping with increasing packet size and the "don't fragment" bit to find where it breaks. Related to Path MTU Discovery black holes. Related to jumbo-frame MTU and MTU.