If the control supports mnemonics, make sure they work correctly when the form with additional controls is browsed and the user sends the appropriate accelerator key. For example, if you are writing an ActiveX tab control with customizable captions for each tab, the user could enter in "&Foo" for the first tab, and "B&ar" for the second tab. Verify that sending an Alt-F and Alt-A switch the tab selection in the control. Even if the focus in a textbox control on the form, the ActiveX Control will get first crack at handling the accelerator before Access does to allow say for instance the File menu commandbar to drop down in the Alt-F case. Also, arrow keys in both the Tab Control and the Calendar Control may perform tasks such as switching tabs or moving the selected day in the control. Verify the keys that the control should handle, and those that are not handled are passed on to the container.
You should also verify that if your control has set any of the miscellaneous control bits like acts-like-default-button, that this behavior is working for your control when interacting with other controls on the Access form.