Sounds like a feature that can quickly become annoying, IMHO. Why not have the issues in their own table, CustID_Fk as the foreign key to tlbCustomers and list the issues in records along with other pertinent details, such as the date of occurrence, type, who logged, etc. and present the issues as a continuous or datasheet subfrm on the main form? The event you use for the other idea could drive you nuts. If it's the form current event, the dialog will pop up for every record where there are issues. User might only want to see other info about the customer (maybe like when they became one) and would get an annoying prompt each view. So you might use a button to show issues instead. Then if I don't click it, what's the point of only revealing such warnings if I think to do so?
If you don't like the idea of showing a subform (or even just a textbox warning there are issues) then hide the subform or textbox if it's null. The idea of not realizing something is wrong without taking action to reveal it, or revealing it in a way that requires interaction doesn't sound great to me.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.