A method of selecting where error detection circuits should be placed within an integrated circuit uses simulation of a reference and test design with errors injected into the test design and then fan out analysis performed upon those injected errors to identify error propagation characteristics. Thus, registers at which propagated errors are highly likely to manifest themselves or which protect key architectural state, or which protect state not otherwise protected can be identified and so an efficient deployment of error detection mechanisms achieved. Within an integrated circuit output signals from inactive circuit elements may be subject to isolation gating in dependence upon a detected current state of the integrated circuit. Thus, inactive circuit elements in which soft errors occur have inappropriate output signals gated from reaching the rest of the integrated circuit and thus reducing erroneous operation.