S::P::Player::fade_volume doesn't call its registered callback if it stops the fade because it detects a manual volume whilst it is doing a fade.  S::C::Commands::mixerCommand calls fade_volume but asks it to do a callback to _mixer_mute when it's done.  If volume is altered during this fade period, the callback doesn't happen and so the mute isn't completed and the player carries on playing, despite reporting itself as paused.

This is unlikely to be triggered by the user altering the volume as fades happen quickly.  However, it can cause some weird interaction issues when things do stuff programatically during the fade period.