Tal, my wife ran into a weird situation today:

She created a web custom control  with very simple text output(”Hello world”), After compiling it, she created a new web application and added the compiled control to the toolbox, and then dragged it into the designer. Oddly enough, the designer simply marked the control with a gray box saying “could not create control” with a tooltip saying “Security error”. Very weird. It worked on all other machines except hers.

After a full day of trial and error she discovered the reason for this. Her .Net framework security setting did not allow that control to appear on the designer. To fix it:

• Control Panel->Administrative tools - > Microsoft .Net Framework 1.x configuration
• My computer->Runtime security policy-> Click the “Adjust Zone Security“ link on the right pane
• Select “Make changes for this computer/user“ as you like and click next
• Select “Local Intranet“ and make sure the lower bar is at least one level below “Full trust“ level

That should solve it. Phew.

And yes, I'm a .Net developer married to a .Net developer. And you thought your familiy was geeky. Heh.