I do iPhone repair work here; had a client supply me with their own replacement back for an iP4S rather than letting me supply. When I fit it, and then removed it a little while later (testing) I could smell that tell-tale sweet smell that means "electrolyte is leaking", didn't think too much of it because a lot of new lithium batteries have it slightly. When I placed the rear panel back on again the phone shut down... and THEN I smelt the all too familar frying electronics magic-smoke-escaped scent, the battery was p*ssed and the phone was about to go up in smoke!
In the end, the phone was saved; the fault was that the customer-supplied rear panel was an ultra cheapy and the die-punch did not cleanly cut out one of the holes in the embedded metal shield, leaving a raised burr. That burn cut in to the battery, released the electrolyte and started a nice bit of trouble. Close call really, another minute and it'd likely have been a pile of ash on the pile of ash that would be my workshop.