Tried to check out a midi file on an Acer Tablet just to see what would support that format. Opened the file using ES File Browser - which apparently has a built in player.
Whatever, no controls like stop, pause, closing ES File explorer didn't stop it.
Had to shut off the tablet.
It could have happened on the Nexus S. Update to ICS changed the link to whatever app played MID files. I use MID for ringtones.
You might also want them to close just in case someone else picks up your phone.
My Vulcan has accidently opened no close apps when I handed him the phone to show him something. Suppose I was searching for his birthday present?