When that geeky kid asks the second guy to get him some Harper's, the guy goes in & steals the the booze & tosses it to the kid. The slip-up is that the booze is in a paper bag. So he stole the booze, then asked the clerk for a paper bag to put it in?
That's cool, but the guy who stole the booze stole money as well. He probably got the owner to bag everything and open the till, then robbed him and ran.
P.S. The geeky kid was (and is) Charles Martin Smith. Look him up at IMDB - he's got lots of great movies under his belt.
Yes, he could have easily stolen the booze in the paper bag. In that store, it appeared that you had to ask the clerk for booze instead of getting it off the shelf yourself. So here's what I think happened, which we don't see. He went into the store, asked for the bottle of booze. The clerk gets the bottle, puts it in a paper bag, then says "That will be $1." (or whatever price). The guy then grabs the bag and runs out.
Another likely option is that the after paying for the booze he pulled a gun or something and robbed the store owner. You see that the robber throws Terry the bottle but the action is so quick you don't see if he has a weapon or not. Also, back in 1962 it is not very likely that a store owner would shoot someone over a $1.00 whiskey.
That's not really a slip up. The guy could have easily stolen the booze in the paper bag. Remember when Terry goes in there to try and buy it? The clerk puts it in the bag before he pays. The guy who stole it probably did the same thing but just took it and ran before asking for anything else. I think Ray has the story down right.