The post-buckling behavior of continuous elastic systems under interaction of two symmetric stable or unstable modes, is considered. For the cases of two stable (unstable) modes with a negative (positive) interaction coefficient, it is shown that mode interaction increases imperfection sensitivity whenever a 2 times 2 energy matrix of the perfect system is indefinite (negative definite). Cases where the simultaneous mode design criterion does not lead to an increased imperfection sensitivity are considered. An example of this situation is supported by experimental results.