[samus]

This is always the tradeoff with arenas. Either the runtime keeps it safe and postpones actually destroying the arena if there are references leading to it. Or it trusts the programmer, which will eventually result in fun late-night debugging sessions. TA indicates that SBCL is rather of the former variety since it describes debugging tools to find references leading from the heap to the arena.

I guess it is best practice to bury arena management inside some sort of framework* and don't fiddle with them in business code.

*: Edit: more likely, a macro