Make or load a sample and save it as example.flac in the Renoise file browser. If you already have a file example.wav in the directory to which you are saving, Renoise will pop up a warning window saying:
This is an error, because even though example.wav does exist, I am trying to save example.flac (which does not already exist). Hopefully the attached image can make this easy to see.
This behavior is present in 2.5rc2, but I noticed it in rc1 as well. (Sorry I didn’t mention it sooner.)