A couple thoughts:
1 A lot of us have leaks in our trunks, so this could cause a lot of moisture to pile up.
2 There is a rod that connects the dial that directs air to the guts of the hvac system. It can fall off, and the knob will still feel like it's doing something. However no air goes to the windshield vents. This happened to me, and I only noticed it when the weather started getting cool enough that I noticed some fogging. Before then I cheerfully moved the knob whereever I thought it should be, even though it was doing nothing.