• Selected a source root in the project view
• Pasted text containing a class with package statement "com.acme"
• Idea create the class in the default package, but class contains the package statement. (That's another bug. Probably the package should be respected, if the paste is invoked on a source root, or else the package statement should be removed.)
• Pressed F6 on the class and tried to move it to "com.acme"
  => IDEA became unresponsive

I had two other projects open in separate frames.