Why does Unity's memory usage increase continuously while the profiler reports static usage?

I'm trying to debug what appears to be a memory leak of some sort. When I start a particular scene in my game (running in the editor on a Win XP box) after maybe 30 minutes sitting still in the scene, the editor crashes with a message saying something like "fatal error in gc too many heap sections".

If I look at Windows Task Manager while the game is running I see the Unity.exe Mem Usage start around 200MB and slowly and continuously build up to about 600MB before it finally crashes. So, I immediately think memory leak. The thing that's confusing me is that I also pull up the Unity Profiler and look at the memory usage in there. It shows a flat 100MB used from the time the game starts to the time it crashes.

I know I still need to investigate my Update() functions to make sure there's no weird object creation going on there. Though, I can't think what I might be doing that the GC wouldn't be able to take care of on its own.

Regardless, I don't understand why I only see memory increase in the Task Manager but not the Unity Profiler. My instinct says that's a leak in Unity itself rather than anything I've done, but I don't know for sure.

Any ideas?