Mysterious tkinter bug: tk.Button doesn't think it passed event <1>

[Ben Elam]

I've stripped things down to the least amount of code necessary to reproduce the error. I can make tk.Button handle event '<3>' but not event '<1>'. That is, the function the event is passed to receives the event and can even print the address of the event object, but a callback later produces an error suggesting that no event was actually passed to the function. Also, tk.Label handles '<1>' without problem. What gives?

Code, errors, and slightly more details here:

http://ingcake.github.io/2015/07/02/tkinter-feature-request---clickable-button/

I hope I'm just doing something dumb; any help would be much appreciated.