Yes, it’s a long word, but sometimes I got the following error message:
Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occured while processing the request on the server. The status code returned from the server was: 500.
On my own I’ve already found the cause: In my subject field I enter a text including words which are recognized as HTML Tags, e.g. ‘Check whether amount <the previous amount”. Then I switched to another tab page and selected a new value from a dropdown list. This is in an Ajax UpdatePanel with Autopostback = True. After selecting a value the error appeared, the script did not reach the SelectedIndexChanged event. The error does not appear when I add a blank after the < less-than, e.g. “Check whether amount < the previous amount”.
Of course I searched the internet and found several reports. Finally I reached a WhitePaper from Microsoft which exactly explains the cause and the fix: http://www.asp.net/learn/whitepapers/request-validation/ It’s not a bug, it’s a feature, a quite useful! But unfortunately the error Message (Status 500) is not very good…. So you could disable the validation and do it on your own, or leave it as it is and live with Error Code 500. In my case it appeared in very rare cases only and therefore I kept the default validation instead of doing it on my own, as this would also effect some other systems accessing the values I stored HTML-encoded then.