Carrier sensing works only when a station can hear the other transmitters whose frames might interfere at a receiver.

In the hidden-terminal problem, stations A and C can both reach access point B but cannot hear each other. Each may sense an idle channel and transmit, causing their frames to collide at B.

Wi-Fi mitigates contention with randomized backoff and can optionally use an RTS/CTS exchange. A sender requests permission to transmit; the receiver's clear-to-send response can be heard by nearby stations that could interfere, causing them to defer.

RTS/CTS adds overhead, so using it for every small frame would be wasteful.

The deeper lesson is topological: in wireless networks, “I cannot hear another sender” does not imply “my transmission cannot interfere with it at a third receiver.” Medium-access decisions must account for receiver-side contention.