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Here's an example of the type of error which is discarded by
Tkinter._tkerror:

from Tkinter import Button
b = Button(command="expr 1/0")
b.pack()
b.mainloop()

When "b" is pressed, "expr" will produce an "Error: divide
by zero", and the equivalent wish app will show the bgerror
dialog.  But replaces the normal bgerror handler with one
that discards the error, doing nothing.

Of course, I've pointed out in an earlier comment how
idiscovery can subclass Tk to handle this kind of error.