Just encountered my first 414 error: “Request-URI Too Long”. However I’m surprised it hasn’t happened to me sooner given how many Web 2.0(TM) websites use api calls to other apps via urls. (In other words, daisy-chaining ever longer referring urls into the next url visited.) For example, this is the url even the mighty Google baulked at when I wanted to post a comment on a blog by logging in to one of the main social media services linked to from within the blog page itself (thanks, TypePad).

https://www.blogger.com/openid-login.g?oidrp.identity=http%3A%2F%2Fthese-are-testing-times.blogspot.com%2F&oidrp.return_to=https%3A%2F%2Fwww.typepad.com%2Fsecure%2Fservices%2Fsignin%2Fopenid%3Farchetype.to%3D%252Fsitelogin%253Ffp%253Dc635b981cc45b06c0d2fd7cb720da2c5%2526view_uri%253Dhttp%25253A%25252F%25252Fprofile.typepad.com%25252F%2526service%253Dopenid%2526uri%253Dhttp%25253A%25252F%25252Fmarkhadfield.typepad.com%25252Fthat_gormandizer_man%25252F2009%25252F02%25252Fthetrainlinecom-fail.html%26openid-check%3D1%26archetype.via%3Dblogside%26tos_locale%3Den_US%26portal%3Dtypepad%26archetype.signin_openid%3D1%26oic.time%3D1294411291-e889c028716a401f72b9&oidrp.trust_root=https%3A%2F%2Fwww.typepad.com%2F&oidrp.assoc_handle=&oidrp.sreg.opt=timezone%2Cdob%2Cgender%2Clanguage%2Cpostcode%2Cfullname%2Ccountry&oidrp.sreg.req=nickname%2Cemail

What do you mean you can’t process this? It’s only 844 characters long!

[Edit: 10/01/11]
Further discussion here; 255 characters seems to be the gentlemen’s agreement.