I have guests at the hotel who went from Amman to the KHB by midday and found it closed .Apparently there were too many busses of Amras (Muslims leaving Palestine and Israel for Saudi Arabia to perform the Amra)
It is wise to check with a good travel agent or call the Jordan side of the border to check on the closing time of the borders.
The Amra season can last till end of Ramadan late Aug
cheers

One more thing, at least two years ago it was highly recommended, if using the King Hussein / Allenby Bridge, to leave very early in the morning from Amman, as queues can be very long as time passes. In 2010 I have left at 6am from Amman and still the entire process, from the Jordanian border to entrance into Israel, has taken some three hours (!) but definitely, people should be there well before Midday. At least these are the suggestions that were given some time ago :-)
Silvia Masiero
Israel and the Palestinian Territories - Hiking
princess actually it dsnt take long time(15 minutes) at all at the Jordan side of the border.People are being held up and delayed till such a time the bus get the OK from the other side to cross the bridge.It is always the case.
cheers.

Marcopoloo - sure you are right, I have only been through the border once, my experience of it is very limited... in effect, there is more queue at the Israeli side of the border, that is the longest part of the process. But still, it seems it's recommendable to go early. I'm sorry for your guests that found it closed :-(
princess thank you for adding the information /your first hand account to this important subject
cheers