Keystrokes in Winforms app causing window to close unexpectedly

I have a strange issue that has arisen recently:

Whenever I enter text, even a single character, into a textbox in any Form in my application it causes the form and its parent to close. I've checked for the following so far:

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• Errant/mis-assigned event handlers that may be interpreting a keystroke as a Form cancel
• I am using keypreview in several windows but debugging shows this to not be a cause
• Happens in any form of the application
• Happens even with brand new text boxes dropped on the form
• Tried removing the WithEvents declaration from text box declarations (VB.NET)
• The result is DialogResult.Cancel when I break the code after Show or ShowDialog
• The forms do not use AcceptButton or CancelButton properties (set to none)
• Note: I am modifying a large codebase with a lot of code that I have yet to touch

What else could be causing this strange behavior?

• Press Ctrl+D, E (Debug, Exceptions) and tell Visual Studio to break whenever any exception is thrown, then see if there are any exceptions at work.

• Press Ctrl+D, N (Debug, New Breakpoint, Break at Function) and set a breakpoint on System.Windows.Form.OnClosing, then look at the call stack. (Alternatively, override OnClosing in the main form, then set a breakpoint in it)

The DialogResult of the button was set to 'Cancel" by accident. After change back to default value "None", the problem is solved. This is my story.