For example;
Citadel: Barla Von
Barla Von has provided the location of Shadow Broker forces trapped in the Dranek system, inside the Krogan DMZ. Recruit them to the war effort and return to Barla Von.
Except that, there's no Krogan DMZ yet
Citadel: Heating Unit Stabilizers
On a wiki site: "Some people might have to make it farther into the game to be able to buy this. (It is a bug, probably)" Meaning I have to just keep checking at random intervals
Irune: Book of Plenix
A volus on the Citadel needs the Book of Plenix to inspire his people. Recover it from Irune and return it to him in the Presidium Commons.
Although you get this mission pretty early on, you won’t be able to unlock the Aethon Cluster, in which Irune is situated, until you advance the main storyline.
Citadel: Hanar Diplomat
Salarian Spectre Jondum Bau suspects that a member of the hanar diplomatic staff is indoctrinated. Find evidence on the hanar in the Presidium Embassies.
Got the first piece, went mad looking for the others, wiki says you need to progress the story to unlock the trial of other devices, fun... so I have to keep checking now and again.
See where this is going? Constantly picking up mission, no indication it CANNOT be done, go to where it's supposed to be but the item isn't usable, the system isn't there, or it isn't but isn't highlighted for a quest, or it is there but the particular planet is hidden until you progress the story.
It's counter inuitive from the first two games and from the rumours of it being a short game I assume this was an artificial method of extending the games life sending people on fools errands until they figure it out.
Worst thing is, if you progress the main story too far you will FAIL these side missions, meaning you have to check between each story arc to see if they've been unlocked.. yeap, making it longer than it needs to . Even IGN noted this in their review :|