https://tinypic.com/view.php?pic=23rw4m&s=6
After stopping elevator from falling those NPCs stand there in that rather funny pose. 
After going up (and loading) though, elevator is fallen at the bottom but barrell stays still at the place.
Same happens if you go up before elevator falls (in both cases elevator button hasnt been pressed).
Thats so ingenious! Thanks!, I need to remember to do that, I always felt so sad for those poor scientists.
Yeah you werenāt intended to be able to save the elevator. Until now Iāve found Black Mesa to be a great case of āthe dev team thinks of everythingā but clearly thatās one thing they didnāt think of!
Thatās actually a recurring problem in quite a few places: Physics objects will get stuck under and between moving objects and cause them to jam. I exploded one US Marine turret, causing a blast door in āWeāve Got Hostilesā to self-seal in the process, and the turret fell under the door. This prevented it from closing completely, yet also prevented me from using the emergency button to open it and made it impossible to get through.
I think, if at all possible, that large objects like lifts, blast-resistant gates and so forth should be able to crush physics objects even if they are normally indestructible. Either that, or all physics objects should be destructible. Itās all too easy to bump into a barrel, knock it down an incline and have it roll into wall mines, explode them and cause a blast door to close down on top of it, making the game unwinnable.
Just let them die 
Pull that barrel and let them fall.
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valveās Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.