Mass Effect 3 Disappearing Citadel Missions

Hopefully, you've been warned that certain Mass Effect 3 side missions will disappear if you don't play them right away. So far, I count four. "Grissom Academy." (Don't miss this one, if Jack is still alive after ME2. Good to see Jack again.) "Tuchanka Bomb." "Ardat-Yakshi Monastery." "Rescue Cerberus Scientists." Now if you fail to finish certain Citadel missions, those may also disappear.

So far I've lost two Citadel missions. There was the "Missing son" mission where you look for Ambassador Osoba's son. And yes, I found out he was dead by picking up his dog tag in the "N7 Cerberus Abduction Mission." Came back to the Citadel and the Ambassador was gone. And the other one was "Ismar Frontier: Prototype Components" mission. The salarian who wants those components disappears after awhile. I'm sure there are a few more but I completed them.

There seems to be one triggering event. It's the "Cure the Genophage" Priority mission. If you complete that mission, it will cause the "Citadel" priority mission to be played if you try to land on the Citadel. That mission is where the Citadel is under attack by Cerberus. Once you complete the Citadel mission, everything changes and you can collect new assignments on the Citadel.

Here' my tip and I'll update this post if it has negative consequences. Complete as many side missions, N7 missions and Citadel missions as you can before you start the "Cure the Genophage" mission.