What can cause errors in how a form displays?

The scenario where removing a source control without updating the conditional expression leads to errors in how a form displays is due to the dependency relationship between controls and their conditions. When a source control is removed, any conditional expressions that referenced this control will likely become invalid or produce unexpected results. This invalidation may result in controls that should be visible or formatted in a certain way not rendering correctly on the form. It highlights the importance of maintaining consistent references in conditional expressions to ensure that the UI behaves as intended.

In contrast, while using a new theme can potentially alter the appearance of a form, it doesn’t inherently cause display errors if the underlying logic remains intact. Adding multiple controls at once is generally a supported action and should not cause errors unless specific conflicts arise that are directly attributable to those controls. Setting all controls to be hidden is an intentional design choice that would not be seen as an error; rather, it would simply result in a form that does not display any controls. Therefore, the removal of a source control without proper updates is the most direct cause of errors in form display, making it the correct choice.