We present an approach for finding errors in programs and specifications. We formulate our approach as an execution mechanism for a non-deterministic guarded-command language. Guarded commands have already proved useful for verification-condition generation but are usually viewed as a non-executable representation. We show how to execute guarded commands using an explicit-state model checker. We illustrate the benefits of this approach in two related domains: bounded-exhaustive testing and falsification for Hoare triples. The basis of our approach is the delayed-choice technique for improving the execution of guarded commands. Delayed choice postpones non-deterministic choice of values until they are used. Our approach also supports copy-propagation of symbolic values but avoids the cost of full-blown symbolic execution. We describe an implementation of our approach in Java PathFinder, a popular model checker for Java programs. Our experimental results show that our techniques significantly improve performance compared to the current execution strategy in Java Pathfinder.