Soda water is not an acceptable substitute for lemonade. I ordered lemon-lime and bitters, I was expecting to get lemon-lime and bitters. What I got was soda lime and bitters, as did several other people on my table. People who ordered raspberry lemonade? They got raspberry in soda water. People who ordered lemonade? Soda water, straight.
I'm... really not sure how you managed to screw up this badly, guys. I get that the majority (as in all but 2) of you are trainees, but seriously, this is beyond basic.
If you were out of lemonade, that's fine. You came up to inform the people who ordered coke when you ran out of that, so why wouldn't you say anything? I would have ordered something else (certainly nothing with soda water in it, since I hate the stuff). The only other reasons I can think of for why this would happen is if you just thought we wouldn't notice/care that you'd given us the wrong thing, or that someone brought out the wrong bottle and nobody behind the bar noticed they were using the wrong thing (I guess it's an easy mistake to make if you take the labels off...?). If either of these are the case then I really have nothing to say.
I hope your training proves useful.