This is now my first saveload or the first de facto lost mission since ages.
It happened on (guess what) Old mine, which is a known pain in the ass anyway since most of the time all of those civies which are not on the surface have little chance to survive and there's no way to help them.
The mission was running about normal, I had lost 2/4 cannon fodder, 3/4 civies dead (normal for this map) and 2/8 aliens remaining.
Near the end of my turn my scouting cannon fodder happened to spot an alien when I had little TUs left. I decided to try rescuing the cannon fodder by blocking the alien's sight with smoke. In hinsight this was the main mistake. I should instead have let it die, that perhaps would also have wounded the killing alien with reaction fire as I had enough TU left for that (but not for a kill). Flashbanging was also impossible. The alien killed my cannon fodder anyway, despite having to move through both fire and smoke. In my next turn I had to search for the alien, moving through the smoke causing more suffocation to my remaining units. If IR sight wouldn't be blocked by smoke I would have discovered it. But now I decided...okay, pointless, not going to find it. I spotted the other alien and killed it. Moved my last cannon fodder into a position so that it would be the unit the alien would likely kill its next turn, retreated the other (valuable) soldiers. I wanted to throw a fire grenade then, roughly onto the area where I suspect the alien would have to move through to reach my smoke covered soldiers. Distance 4 fields, line green, enough space around the green line...blargh. It hit the crate I wanted it to throw over and exploded exactly where it should not, stunning 3 soldiers in return, 2 of them valuable. As they were also on fire there was no way I could have helped them, they suffered fire damage each turn and died. I had 2 soldiers left, one of which was on the surface, and the other also 1 or 2 turns away from being stunned. Since the last alien was bleeding 1 points per turn I could have waited a very long time to still win the mission but shamefully decided to retry.
The point is that if it was either not for suffocation, or not for IR sight being blocked by smoke, or even not for both of these, this would not have happened.
I also think grenades (thrown, rolled or shot) should be somehow more precise in general. It can be applied to aliens (plasma grenades) as well. Using grenades is too much of a luck issue for a strategy game. The green line always tricks you into believing this would be safe, but beware if there is any kind of fence or similar obstacle it is guaranteed to be hit, don't even think of it.