Is it bad to marinate meat for too long? From my experience it is always best to marinate meat for a decent amount of time, so the meat can "soak" up the marinade and be more flavourful.

However, I came across this recipe here: http://www.abc.net.au/tv/cookandchef/txt/s2264630.htm

In the directions, it says: Add pork and marinade for a minimum of 6 hours or overnight, but no longer than 12 hours.

I was wondering, why does it say "no longer than 12 hours"? Isn't it good to let meat marinate for as long as possible?

Thanks.
Two things-

If the marinade is very strong or salty then the meat could simply become over flavored.

If the marinade includes a meat digesting enzyme such as papain then leaving it too long could turn the meat to mush.