Currently, PROTEST/BASE:DEFINE-PROTOCOL-CLASS defines a method on INITIALIZE-INSTANCE. This is buggy, since this means that it is still possible to create instances of the protocol class by e.g. using CHANGE-CLASS. This means that, in the future, DEFINE-PROTOCOL-CLASS will also define methods on UPDATE-INSTANCE-FOR-REDEFINED-CLASS to prevent such.
This issue is a warning to all clients of this method to ensure that they do not have. It's unlikely to have them, since changing classes to/from protocol classes is a weird thing to do, but I nonetheless post this here.