Sometimes Tux gets stuck in sprint mode.

Holding ctrl down makes him walk temporarily, but as soon as I let go, he starts 
sprinting again.  I suspect some sort of internal state and/or keystroke 
monitoring is off.

It's actually the opposite way around, and 
you can toggle sprint mode by pressing "U" 
once.

If you are on sprint mode (pressed U 
once), Ctrl will slow you down and allow 
you to recover Stamina.

Sprint mode is persistent between 
savefiles.

No further comments on this topic, I'm assuming the problem was resolved.
(ie. I'm assuming that was a false-positive because the U key effect persist
between savefiles)

Therefore, closing the issue.
Please reopen if you believe this is not the case (and indeed, there's a bug
with the U key not working as it should).