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:

• 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?