I racked my cider after primary intending to bottle it, when I realized i didn't have enough bottles. My nearest homebrew store is a half hour away and I don't have a car, so it was a couple of days before I could bottle.
When I opened it up to prime, there was film yeast http://www.cider.org.uk/part5.htm
on the top, probably because I left too much headspace since I wasn't intending to let it sit more than a few minutes originally. To be clear, there was far, far, less residue in mine than in the linked picture. I'm talking a millimeter thick or less. Sadly, I had to stir it because it wasn't primed yet, but when I did, the film turned into a golf-ball sized piece of foam that looked exactly like the foam you get when oxygenating beer before bottling. I scooped out as much as I could, but didn't get it all.
After some research, it looks like oxygen deprivation can work to suppress film yeasts. To me, that seems like in theory the CO2 produced while carbonating would be enough to kill the yeast, but then again, there must still be enough oxygen in there for the good yeasts to do their thing and carbonate the beer.
I guess there's not much I can do now, but it would be nice to know whether to expect vinegar or not when I open the first bottle. Does anybody have any experience with this?