An interesting quirk
I recently discovered (completely by accident) that if you have negative cargo space due to the popular "mass trick", any mission offered in the Bar (and, presumably, the Trade Center, the Shipyard, the Outfitter, and the Main Spaceport - BBS is fine) that has cargo at all (even weightless) will not be offered, even if the mission has 100% availability and the player meets every other requirement.
Not sure if this has been mentioned before, but I found it rather interesting. It also means that using the trick is a bad idea mid-string.