Weird error [htmlfile: Unknown runtime error]

I was creating a very simple page to play around with the Update Panel when I came across a weird error.

The page uses an Asynchronous post back trigger on an update panel to control the updating of the panel. Seems simple enough but I ran into trouble. The code I tried to run is below.

When I ran the code above I received a weird error from Visual Studio.


I did some digging and found the offending code to be the paragraph tags “<p>” and “</p>” inside the ContentTemplate tags. Removing the paragraph tags allowed me to run the code without error and continue to play!