ooh, Excellent topic!
I'm sure everyone has made at least some minor boo-boos (even if they don't say so).
A LONG time ago, when I was still a young babysitter (maybe 14 or something), I accidentally let the indoor cat out the door and the kids and I had to walk the neighborhood and find the kitty for our afternoon walk. It did turn out to be a fun adventure and we did find the cat, but boy was I scared at the time.
When I called and told the parents (before we went on the search because the cat ran fast and was way out of the yard very quickly), they were understanding and actually apologized to me for not telling me that their cat was somewhat of an escape artist and was always trying to get out. LOL
But, some people may not be as understanding, so don't forget about the pets, too.
Not a mistake exactly...just a personal rule....NEVER make a promise to a child...because if you don't follow it up, they will remember. I mean little things like, I promise we can make the picture after tea", because if you don't and then bedtime comes, you're guaranteed to hear "I'm not going to bed til we make the picture...you promised." I think this stems from hearing broken promises when i was a kid...i just don't do it anymore, not with little or big things. There's nothing worse than being let down for a kid and whether its that promised picture or a trip to the pool, they'll always remember it and connect it with you.
Secondly, don't trust the kids with too much...once when i was younger i forgot to ask for a bedtime so had to rely on the eleven year old to tell me...this backfired when i heard the parents drive up while the kids were still in the lounge chatting...oops...i should have known she'd tell me she was allowed to stay up much later than she actually was...