The sort that logs a warning in latest.log but lets the script carry on.
I can see some scratching chins as to why a notification that something you are trying to invalidate is already in that state is necessary, but if the function could do one of several things to said entity depending on circumstances it might be handy to remind scripters (i.e. me) to include appropriate validation.
A warning in latest.log is more than I get for the sort of problems I have.
(These are mostly self-inflicted due to a typo or logic error on my part, so I won't trouble you with them.)
Still anyone who is writing an OXP script should be checking latest.log both for errors and warnings. If not, the community may well correct them.
Timing of a removal can be tricky if something else kills it at that same moment. Would something like an if(entity != null) check before removing it help in your case?
To be honest, I think the best behaviour here would be it returning false if the entity to remove couldn't be removed because it was being removed already.
IIRC, that's consistent with a number of other functions...