Interesting story in the Missed Apex podcast from ex-Lotus boss Matthew Carter. Here's the video, the story starts at 27:10.
2015 Spa, Grosjean was chasing Vettel for P3. Mercedes PU engineers told Lotus to put the engine into a certain mode they had never shown them before. Grosjean said the car changed entirely and he would've caught Vettel even if it wasn't for Vettel's late puncture. Lotus asked Mercedes what it was, Merc said nothing, and they never allowed them to use that engine mode again.
We knew about special engine modes, extra Q3 power and all. What's news to me is that those modes are 1) available in customer cars as well, 2) but it doesn't mean they're allowed to use them unless authorized to.