The story: Things started out with Mara teaching some of the in-period forebears of longways dance, because while Harald had brought the computer and the boombox and everything, he'd left the music itself at home, and had to run back and get it. He got back a little later, and taught a slightly complex triple minor (The Lamplighter's Hornpipe, IIRC) from a printout -- that had several stops and starts, having to tweak the teaching as we realized that things didn't quite work as expected. It took a good while, mostly because of having to go back and fix things several times. Afterwards, he taught a dance he knew better, which he'd put off because it had a slightly complex figure (the Ladies' Chain), which actually proved to be pretty easy to teach: he knew the figure well enough to teach it smoothly, and it's very close to a figure in Parson's Farewell, a local favorite dance.
There are three lessons here, all of them sort of obvious in retrospect but all of them easy mistakes to make and worth keeping in mind:
Checklists: It's very useful to review what you'll need for running dance practice well in advance, and to write it down. Having it in your head is no substitute for having a post-it note on the door. Even when it's just one dance bag, I try to keep in the habit of putting a note on my hat the night before -- I've walked out in the morning without my dance kit way too many times.
Know the Choreography: The problem with Lamplighter's Hornpipe was that on paper it looks terribly easy. But what that actually meant was that it was very ambiguous. Learning a dance from paper is always a process of reconstruction, regardless of whether that paper is period or modern -- indeed, many modern sources are worse than the period originals. So if you're teaching a dance that you don't know intimately, it's very important to work through it in considerable detail beforehand, to make sure you find the gotchas. (For example, in this particular case if you follow the directions literally, you wind up with the active couple improper, which is unlikely. So we had to figure out on-the-fly how to patch the choreography to fix that.)
Very Little is Actually Hard: SCA dance teachers often distinguish very strongly between "easy" and "hard" dances, but it's important to bear in mind that that's at least partly a fiction. Most SCA dances, even the ones that most people consider "hard", are actually pretty easy to teach if you know what you're doing. (Yes, there are some genuinely hard dances, but they're mostly from Negri and rarely done in the Society.) The difference comes mainly in how well the teacher knows the dance himself, and in what the students already know. In this case, the second dance was actually much easier than the first, not because of the complexity of the dance itself, but because the teacher knew it better and because it mapped a little more closely to typical ECD. The difficulty isn't the dance itself -- it's how it is taught and contextualized by the student's prior knowledge...