My pet peeve is sites that switch me to a mobile version because I'm using my iPad.
Listen up: that MIGHT make sense on an iPhone, but on a 10 inch tablet, I'm usually much happier if you'd just let me use my browser and not try to "optimize" my experience.
You aren't optimizing it, you are usually making it worse - and in any case, it should be MY choice to switch, not yours!
Yes, I know: I can use a different browser that would allow me to change the User-Agent string. Yes, I know that half the people who complain about this blame Apple for not letting us control that in Safari.
There's also this possibility: http://www.technipages.com/ios-change-the-user-agent-in-safari.html
But I blame the jackass web designers who don't understand that people should have a choice.