Button click event handler fires twice

I had a simple button on a form together with its click event handler. On debugging, I noticed that the event handler was firing twice. After a long investigation, I decided to check the HTTP headers. These revealed that the "Display Page Validation" option I was using in the Tools menu of the Web Dev toolbar in Firefox was the culprit.



When switched on, this runs the handler a second time, submitting the results to a validator at w3.org. This is something to bear in mind, particularly for code that must only run once, such as handling credit card payments.