If you're using the .NET Framework 2.0 with Visual Studio 2005 and ASP.NET AJAX 1.0, you may run into odd errors when you use the Wizard control and you try to embed an Accordion control inside a WizardStep.
When I did this, it was all in the ASPX code view and the code-behind file. The page and the project built cleanly and the run-time tests were fine, as well. But when another developer then tried to switch the ASPX to design view he saw an error for the entire Wizard control that said "Error Rendering Control" and then the project failed to build throwing errors complaining about the Accordion. We carefully tested this to make sure no other edits were made, that we could reproduce it, and that nothing in our configurations changed. When we reverted that file back to the original, the project built cleanly again and worked fine when deployed.
After some searching, all I could find was one post on the http://www.asp.net/ AJAX forums that said it was a known issue in VS 2005 that was fixed in VS 2008. We haven't purchased VS 2008 yet and we're not prepared to upgrade to Framework 3.5 on our production servers yet either, so the suggestions here are not that helpful.
If anyone has a fix for this using Framework 2.0 + VS 2005 + ASP.NET AJAX 1.0, I'd love to hear it.