Double postback issue

It's a very specific problem with this code, I doubt it will be useful for someone else, but here it goes:

A check was added to the combo's onchange with an if, if the condition was met, an explicit call to the postback function was made. If the combo was set to AutoPostback, asp.net added the postback call again, producing the two postbacks...

The generated html was like this:

[select onchange="javascript: if (CustomFunction()){__doPostBack('name','')}; __doPostBack('name','')"]