Worst of those I encounter regularly is US sites trying to constrain your input to numbers only on certain fields.
Not "letting you input anything and marking it invalid if you put something other than a number", but listening to which key is pressed to allow the typing or not.
On French keyboards, typing a number requires the use of the SHIFT key. So I regularly need to change my keyboard layout just to type a couple numbers in a form field because someone thought they'd be smart...