I don't know if this is the same problem as everyone else has seen (sounds like maybe it's different).
I wrote a page with a chart in it.
I then changed the page to use an asp:Wizard - the chart being in the last step.
After this change, I got the executing error shown below when I reached the last page.
Moving the chart out of the wizard and back into the root of my control removed the error.
However, when I moved the chart back to the last step of the wizard again, the error was cleared....
Somewhat confused at present...
Error executing child request for ChartImg.axd.Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Web.HttpException: Error executing child request for ChartImg.axd.
[HttpException (0x80004005): Error executing child request for ChartImg.axd.]