I have a fill clock running a /entitydata command to change a mob's equipment if they have a objective score for "Category" of 1. When I press a button, two commands are executed: First it summons the entity (Skeleton with Armor and other tags like PersistenceRequired:1), then it sets the Skeleton's Category to 1. It is then killed. The Skeleton does appear for a fraction of a second before dying. Disabling the /entitydata command causes the Skeleton to be summoned without issue. Activating the /entitydata command by hand or by clock after it is summoned causes it to change the Skeleton's equipment exactly as expected with no issue.