The article An Error at Run-Time due to the Database Connection Left Open at Design-Time explains why sometimes live database data at design-time can cause problems at run time.

In short, when the application is started, if the Connected property is left to "True", at design-time, the application will try to connect to the database using the "hard" coded ConnectionString property!

TAdoConnectionEx

Here's the full source code (yes, it is that small) to a custom TADOConnection derived component.
The TAdoConnectionEx is an enhanced ADO connection component that prevents design-time database connections from being used at run-time unless explcitily allowed via the AllowStreamConnected property.


unit ADOConnectionEx;interfaceuses ADODB, Classes;{   Enhanced ADO connection component that prevents design-time database connections   from being used at run-time unless explcitily allowed via the AllowStreamConnected   property.  Author: Larry Hengen } type   TADOConnectionEx = class(TADOConnection)   private     fAllowStreamConnected: boolean;   protected     procedure SetConnected(Value : boolean) ; override;   published     property AllowStreamConnected : boolean read fAllowStreamConnected write fAllowStreamConnected;   end;procedure Register;implementationprocedure Register; begin   RegisterComponents('delphi.D106', [TADOConnectionEx]) ; end; procedure TADOConnectionEx.SetConnected(Value : boolean) ; begin   //if we are not to allow a connection to be established   //when reading in the form, then   //toggle it to false when Reading   ifnot(FAllowStreamConnected) and (Value) and (csReading in ComponentState) then     inherited SetConnected(False)   else     inherited SetConnected(Value) ; end;end.

Basically, the SetConnected method - a protected method that gets called when the public Open method is used.

Installing into the Component palette

The TADoConnectionEx source code above, should be saved in a single unit file (ADOConnectionEx.pas). You'll need to add the component into an existing package. Here's How to Install Custom Component in Delphi (into Existing Package).