We had a flight once where we knew we probably wouldn't get in (socked in coastal city), so we discussed it with the dispatcher and set up an alternate (which they presumably told), weather warned the passengers, put on a crap ton of hold fuel, and took off. When we finally hit divert time, we called up ops at our diversion airport only to be told that everyone except the manager had gone home and we shouldn't come there, couldn't we go to XYZ instead? Ummm, no. We only have enough fuel to land at your place, so we'll be there in 10 minutes.
It happens, even with the best laid plans.