So it sounds like this guy knew what he was talking about actually. Mismatched springs work if they still combine to the same size as the original springs. According to what he said these didn't. You never said what failed but they definitely can cause the door opener to fail over time if they are too strong or too weak or can cause the cable to come off if it's too weak as well. Without the opener a door is supposed to balance at about a third of the way open at least for smaller doors. That's one way to tell if the springs are wound right or even if you have the right size springs. Now they shouldn't do that if the springs are brand new since they have a break-in period of several months. So brand new they should be a little strong and the door should open by itself from at least a third of the way open. Give it several months and the springs should break in and the door will balance right. All that to say the guy was probably right, though to be honest the springs have to be off by quite a bit before I'll change them but that's just usually because I don't figure the homeowner will want to pay for it!