So I cannot find any reason I am seeing the below behavior and if anybody has some insight it will be greatly appreciated.
Basically I am using the FB.UI from the JavaScript SDK to send a message to a user with a link. The link ends with a Guid, like http://www.domainname.com/register/33a1a0ae-e0fe-4eb6-9bf9-146d5492e3d6. This works sometimes, but occasionally fails with a generic 500 error from FB.
I have pulled out the HTTP POST request and have found a solution that I can recreate, unfortunately I cannot share the access code to allow SO users to actually run it (I replaced all sensitive parameters). Below are two identical requests that differ only in the Guid. The first succeeds every time and the second fails every time. I have numerous Guids that are doing this which makes if unreliable.
https://www.facebook.com/dialog/send?access_token=XXX&api_key=XXX&app_id=XXX&channel=http%3A%2F%2Fstatic.ak.facebook.com%2Fconnect%2Fxd_arbiter.php%3Fversion%3D5%23cb%3Df2cb8f5c1ca0402%26origin%3Dhttp%253A%252F%252Fwww.domainname.com%252Ff350c0fd55d5764%26domain%3Dwww.domainname.com%26relation%3Dparent.parent&channel_url=http%3A%2F%2Fstatic.ak.facebook.com%2Fconnect%2Fxd_arbiter.php%3Fversion%3D5%23cb%3Df11a615f3b71192%26origin%3Dhttp%253A%252F%252Fwww.domainname.com%252Ff350c0fd55d5764%26domain%3Dwww.domainname.com%26relation%3Dparent.parent&description=test&display=iframe&link=http%3A%2F%2Fwww.domainname.com%2Fregister%2F**33a1a0ae-e0fe-4eb6-9bf9-146d5492e3d6**&locale=en_US&name=test&next=http%3A%2F%2Fstatic.ak.facebook.com%2Fconnect%2Fxd_arbiter.php%3Fversion%3D5%23cb%3Df22e359d88321ce%26origin%3Dhttp%253A%252F%252Fwww.domainname.com%252Ff350c0fd55d5764%26domain%3Dwww.domainname.com%26relation%3Dparent%26frame%3Df33c13cd4ecc156%26result%3D%2522xxRESULTTOKENxx%2522&picture=http%3A%2F%2Fwww.domainname.com%2Fimg.gif&sdk=joey&to=XXX
https://www.facebook.com/dialog/send?access_token=XXX&api_key=XXX&app_id=XXX&channel=http%3A%2F%2Fstatic.ak.facebook.com%2Fconnect%2Fxd_arbiter.php%3Fversion%3D5%23cb%3Df2cb8f5c1ca0402%26origin%3Dhttp%253A%252F%252Fwww.domainname.com%252Ff350c0fd55d5764%26domain%3Dwww.domainname.com%26relation%3Dparent.parent&channel_url=http%3A%2F%2Fstatic.ak.facebook.com%2Fconnect%2Fxd_arbiter.php%3Fversion%3D5%23cb%3Df11a615f3b71192%26origin%3Dhttp%253A%252F%252Fwww.domainname.com%252Ff350c0fd55d5764%26domain%3Dwww.domainname.com%26relation%3Dparent.parent&description=test&display=iframe&link=http%3A%2F%2Fwww.domainname.com%2FFregister%2F**dd171262-dbcc-43c3-b9d1-e37dc53e3520**&locale=en_US&name=test&next=http%3A%2F%2Fstatic.ak.facebook.com%2Fconnect%2Fxd_arbiter.php%3Fversion%3D5%23cb%3Df22e359d88321ce%26origin%3Dhttp%253A%252F%252Fwww.domainname.com%252Ff350c0fd55d5764%26domain%3Dwww.domainname.com%26relation%3Dparent%26frame%3Df33c13cd4ecc156%26result%3D%2522xxRESULTTOKENxx%2522&picture=http%3A%2F%2Fwww.domainname.com%2Fimg.gif&sdk=joey&to=XXX