"An escalator can never break: it can only become stairs. You should never see an Escalator Temporarily Out Of Order sign, just Escalator Temporarily Stairs. Sorry for the convenience"

The quote is stating that an escalator can not truly break down or stop working- it only ends up being stairs. Instead of a sign stating that the escalator is briefly not working, it should simply indicate that the escalator is temporarily stairs, with an apology for any inconvenience. This is an amusing and ironic observation about the (absence of) energy of escalators.