The default storage location for memo(1) output is /tmp/memo/${USER}. Most distributions either have some automatic periodic cleanup, and/or wipe it on restart.

Separately from that:

  - The invocation contains *memo* right in there, so you (the user) knows that it might memoize.
  - One uses memo(1) for commands that are generally slow. Rerunning your command that has a slow part and having it return in a millisecond while you weren't expecting it should make the spider-sense tingle.

In practice, this has never been a problem for me, and I've used this hacked together command for years.