Helpful hints on things not to do if you don't want Yael really angry with you:
Do not call her and tell her there is something extremely urgent-- so drop everything and spend two days doing it-- and then when she has pulled out all the stops tell her, actually we made a mistake and we didn't need that at all, we need this instead.
Do not then, after receiving a screaming email from Yael, apologize and then tell her that a very important meeting has been set up, without consulting her or her calender, but that she must drop everything and attend it. Do not then CANCEL the meeting BUT NOT TELL YAEL so that she a)has paid 150 shekels to a vet to feed small kittens during the many hours she is gone b)canceled other meetings c) not gotten any work done on any of the things that are overdeadline because of the shenanigans last week d) paid for a round-trip ticket to hell with no reimbursement and d) wasted an entire DAY going to a meeting that didn't happen. Oh yeah, and had to leave ulpan an hour early to pick up the cats.
Icing on the cake is informing her that the meeting has been rescheduled, again without consulting her --what, I look like I sit around and file my nails all day? --and then act put-out and like she isn't invested in the project because she can't make it, having agreed to and scheduled to give a talk on that particular day some 3 months ago.
Don't go there. And no, my blog police aren't allowed to delete.