Crash in Message Loop

I found a mysterious bug in the .NET popup form code:

Unhandled Exception: System.ArgumentOutOfRangeException: 
Index was out of range. Must be non-negative and less than 
the size of the collection.
Parameter name: index
at System.Collections.ArrayList.get_Item(Int32 index)
at System.Windows.Forms.ThreadContext.System.Windows.Forms.
UnsafeNativeMethods+IMsoComponent.FPreTranslateMessage(MSG& msg)
at System.Windows.Forms.ComponentManager.System.Windows.Forms.
UnsafeNativeMethods+IMsoComponentManager.FPushMessageLoop(
Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.ThreadContext.RunMessageLoop(
Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at PopupControl.frmMain.Main() in PopupControl\frmMain.cs:
line 122
The program '[1600] PopupControl.exe' has exited with code 0 (0x0)

I'm running 1.0 Framework. To reproduce it just click the btnDrop on frmMain a few times quickly. I have the same bug in a project created based on your work. I'll be trying to fix it in parallel.

Not reproduced on test system here. Does anyone have any ideas?