Most common or vicious mistakes in C# development

Question

What are the most common or vicious mistakes when experienced C++ programmers develop in C#?

Answer 1

Thinking that "garbage collection" = "I never have to worry about object lifetime at all". For instance, opening a FileStream and forgetting to close it.

Or:

1. Allocating a lot of objects
2. Putting them in a big global dictionary (usually after "I know, I'll make a cache")
3. Wondering why the application's memory usage always goes up and never down ("but it's supposed to garbage collect!")