The boundaries of model-checking have been extended through the use of abstraction. These techniques are conservative, in the following sense: when the verification succeeds, the verified property is guaranteed to hold; but when it fails, it may result either from the non satisfaction of the property, or from a too rough abstraction. In case of failure, it is, in general, undecidable whether an abstract trace corresponding to a counter-example has any concrete counterparts. For debugging purposes, one usually desires to go further than giving a yes/no answer (actually, a yes/dont know answer!), and look for such concrete counter-examples. We propose a solution in which we apply standard test-pattern generation technology to search for concrete instances of abstract traces.