This is a bug that I've experienced in iOS 6 and 7, mostly on an iPhone 4.
While listening to a song (a downloaded song because I'm offline), sometimes the volume will get SUPER loud for a split second, then return to normal. It's incredibly jarring and instantly can make a nice, relaxing morning into a wide-eyed, heart-racing morning.
Currently using v0.7.1.23.
EDIT: The volume doesn't raise to maximum, but merely a significant amount higher than the current volume.