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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?

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    The bike was improperly adjusted. If a bike shop is giving you such a bike to test ride, and even hands it back to you without adjustment after the first failure, then you need a different bike shop. – Daniel R Hicks Aug 13 '18 at 2:55
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Excluding the factor of very-very cheap bikes with unknown components, damaged components etc, this particular situation is quite typical for front derailleur which was not set up properly. To be able to shift gears without chain falling off on either side you need to make sure to set up front derailleur correctly, starting with height and rotation angle, and ending with limit screws which are designed specifically to avoid the described situation :) After that you can expose your bike even to several meter drops and chain should stay in place (on a mountain bike, with appropriate rear derailleur), not mentioning just a plain simple ride like you had even without shifting.

Many people hate front derailleurs because setup could be tricky. It took me a while to figure out how to set it up and now I can do it even in the field without a stand. So answering your question, I believe that there is nothing wrong with the bike, but setup was done by somebody not really experienced. One of the ways to see that it's not set up properly is to try to shift front derailleur - it only shifts perfectly if every part of the setup is done well.

Useful links on how to set up limiting screws to keep the chain on: https://www.parktool.com/blog/repair-help/front-derailleur-adjustment#article-section-4

Video version of the article, here they also demonstrate which setup leads to chain falling off:

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    Agreed - I'd say this says more about who setup the bike and possibly what support you might get from this particular LBS. – r00fus Aug 14 '18 at 18:15
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To some extent this depends on the quality or cost of the bike, but generally it should not happen on anything above the very lowest department store level bikes.

I'd guess the front derailleur was set up badly

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    Chains falling off are mainly adjustment problems or weak rear derailleur tension. – Carel Aug 12 '18 at 18:24
  • This shouldn't happen on any properly adjusted bike. Nobody would buy a bike whose chain fell off that reliably. Even the worst bikes tend to have something like Shimano Tourney and Shimano Tourney works just fine. – David Richerby Aug 13 '18 at 8:21

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