Raymond Chen explains it well here: http://blogs.msdn.com/b/oldnewthing/archive/2010/08/13/10049...
Unfortunatelly these types of bugs have a nasty way of showing up in other places where it can be even harder to debug. Long term these bugs need to be fixed otherwise your code will just be a ticking bomb. The worst part is when they combine with other bugs and they become even harder to debug.
You could say that the problem is that people abuse the garbage collector to manage something it shouldn't manage, or you could say that the problem is that garbage collectors are bad at managing anything that isn't memory. (I would lean towards the latter.)