I completed the game (achievement hunt, actually), and put the can in the trash can. I got the "Submissive" achievement. But now, I somehow can't get the "Defiant" one. I throw the can right back at the Combine guy, but the achievement doesn't register.
 
I don't know if I'm doing something wrong, or it's possibly a glitch, but has this happened to anyone else?

When you put the can inside the bin, walk past the guard then come back, take the can back out and throw it at the back of his head.

Nope, doesn't register anything. I tried a bunch of different angles and positions, still doesn't register the achievement :(

I reloaded an old save where I hadn't made the choice yet, then chose to throw it at him.