Cl nopext

Name: cl_nopext
Default value: "0"
Flags: archive

To quote from the FAQ:

FTE has some subtle protocol changes. These changes are subtle enough to not cause problems with connecting, however, demos are recorded with the new protocol and are thus unplayable in other clients. If you need to record a demo in a backwards compatable way, set the cvar 'cl_nopext' to 1.

The incompatability will only be recorded in demos if both client and server are FTE based.