Shift & Play buttons swapped over!

A particularly disturbing thing went wrong with my Yocto last night (during a gig!):

The Shift button swapped roles with the Play button... This meant that during a song, I pressed Shift, and the pattern stopped!

I was under pressure, and didn't understand what had happened. Anyway I tried power cycling the Yocto, and the problem was still persisting.

Oddly, about 5 minutes later, the problem went away, and Play and Shift worked as normal.

Anyone seen a problem like this before? I haven't yet opened up the device to have a look, thought I better post this up first.

Update

So I opened up my unit, and went through and checked all the connections to IC7, and noticed I'd not soldered one of the Mute button legs properly...

While I was doing this I made sure I touched up all the soldering to the pull-down resistors (R43-50), and other related components.

Haven't managed to get the unit to do anything odd since... Hope that's the issue gone!