256kbps MP3 is transparent in most cases, but there are some sounds that MP3 is bad at encoding (notably the sound of maracas and similar rattle type sounds), and you can train yourself to get better at identifying MP3 artifacts. People have reported successful ABX of maracas sound at 256kbps, e.g.:

https://www.head-fi.org/threads/when-is-mp3-transparent-an-a...