When you create a Custom Control or User Control in Visual Studio 2k5, it needs to be able to render its running state with no additional information. I.e. it needs to have a no argument constructor and cannot need extra data passed in at initialization, otherwise Visual Studio 2k5 Designer will die when you add the Control to another form or control, even if it comes up fine in the Designer by itself.
Ok, this one is a big Duh! but i'm sure i'll forget again some day in the future and bitch and moan about Visual Studio hating me and constantly crashing. Basically it comes down to the designer embedding the control and running it. So OnPaint will get called and any timer you have that's not disabled will run. Now if you build controls that require extra initialization data, and the running portions of the control try to use them, you'll get a NullException. Makes perfect sense really, hence the Duh. But of course, you may forget that part, because, after all, the control by itself runs fine and only crashes if the control is added to another control/form. My bet is that the designer is basically a control container and will just discard the exception, but if you add the control to another, the control being added to doesn't handle the exception and complaints.
A cool little side-effect here is that that you can put an animation in a control and that animaton will run in the Designer. :)