I work at a utility. We got called out on a customer complaint to a grocer who had burned up his 3 phase motors.

His service was a 120/240 volt open delta secondary fed from two phases of a 4 wire wye, 12.5/7.2 Kv primary.

When we got there, we read across the "lighting" legs and got 120 volts from A to neutral and 120 volts from B to neutral. A to B read 240 volts, as did B to C.

So far so good. So.....

A to C read 480 volts (instead of 240) and C to neutral read 360 volts (instead of 208).
Neither transformer fuse was blown.

Hint: There had been many outages due to storms a few days before and lots of backfeeding had been going on.