I tested a bike at a bike shop today. I rode it about 30 feet / 10 meters, turned left up a very shallow hill, hit the left (forward) dérailleur and the chain came off (went past innermost front sprocket).
I walked it back to the store. They put the chain back on. I did exactly the same thing again, 30 feet, left turn, up hill, switch gears, chain comes off.
I put the chain on myself this time and then walked it back. The bike shop adjusted it a little and warned me not to go completely diagonal on the chain (left most front, right most back or vice-versa). I didn't try it a third time.
I'm not a cyclist by any means but I've owned bikes in the past as a mostly casual rider and never had one derail the chain that easily.
Is it most likely just a matter of adjusting something, or is that bike (or its dérailleur) probably a lemon (bad)? How would I tell?