Thread:Extension talk:VisualEditor/TypeError: Expecting a function in instanceof check, but got undefined/reply

Do not trust the Special page, it's lying to you. The VE master doesn't work with mw 1.25wmf8, because of changes to some of the core. They are just not updating the version dependency when they need to (can't blame them really, the extension is not done yet). The easiest way to solve this is to try and use the 1.25wmf8 branch of VE instead.