Dehydration of aldoximes
My proposed mechanism is a nucleophilic addition/elimination mechanism without a tautomerism step.

It is all concerted.

The first step, addition, features the oxygen in the (E)-propionaldoxime acting as a nucleophile to attack one of the carbonyl carbons in the acetic anhydride, which pushes electrons the carbonyl oxygen, making it negatively charged.

Then, the negatively charged oxygen strikes back, initiating a chain of concerted cyclic electron rearrangement which cleaves quite a few bonds, resulting in three molecules, completing the elimination step.