1. add a plan say A
2. add an additioanl eligibility on Class Wise OE Rule for plan B (with additional eligibility criteria as - AND if enrolled in Plan A)
3. now delete Plan A

Arrive on the same classwise OE rule
In this case system allowed an user to delete the plan used in eligibility (normal - not rules) - and then when user goes to the menu (class wise OE rule in above example) - system throws a server error