In classic 4D, Tables are typically assigned a default Input and Output form. When inheriting someone elses database and working on it, entering design mode can automatically load a current table. In some cases, the loaded table's Output form has an event triggered or the Form's form method generates an error due to the code in the trigger that is trying to access some data that was not properly loaded. This can be even more frustrating when the trigger is continously called even after trying to break out of the method and does not allow the table to be closed.
The best method to prevent this issue is to find out the proper way to access design mode. In these scenarios, there is typically a menu item or feature implemented that is the intended way to access the design mode.
If the above cannot be found, an alternative is to try to see if the code can be broken out of or completed. If it is an endless loop, then try to bring up the Debugger to stop the Output Form from executing, it may take a couple of tries, but one of the errors should trigger the debugger. When the Debugger Window appears, the errors should stop and the designer menu items should be available. From here, try to select a table that will not trigger any errors. Alternatively, a new Table can be created, or a new output form can be applied to a specific table so that it can safely be selected.