A verification tool in a graphical modeling environment allows the components in the graphical modeling environment to be used to define error or exceptional conditions without adverse affects on the ultimate implementation of the model. In one example, a system presents a graphical interface to facilitate design of a model constructed from graphical blocks; receives an association of a plurality of blocks with a verification subsystem; and performs an operation to verify that the blocks in the verification subsystem do not affect operation of the model external to the verification system.