I noticed that anytime I thought I was clever by using memoization on a function with side-effects, that it in fact could become very complicated very quickly.

I noticed that anytime I thought I was clever by using memoization on a function with side-effects, that it in fact could become very complicated very quickly. It's a bit similar in your situation: What if someone actually wants to send the same message twice? Depending on your codebase it could be hard to debug the issue which will arise. One thing where I do think it is kind of safe is for loading a file into a class instance. The memoization will avoid defining (1) a method to check to see if the file is loaded, (2) a method to load the file and (3) a class variable which points to the loaded file.