My wife's iPhone 6 battery reserve was depleting quickly
When I opened the case to my wife's iPhone 6, I noticed 3 things:
1) one of the threaded anchors from the display cover frame that receives the pentalobe screws was broken off and fell out. It had clearly been crushed at the lower end -- likely from someone previously trying to close the phone without having everything in place as it should be first (my guess is they failed to match up the hinge properly at the top end, so crushed the protruding anchor at the home button end when trying to close the case).
2) the 1.7mm screw to secure the front panel assembly cable bracket was missing, causing that corner of the bracket to be jutting upward.
3) Three screws were missing from the frame of the display and digitizer.
Shortly after my wife had purchased this phone new from AT&T, she dropped it and cracked the screen badly. We had it repaired by a "professional" at Batteries & Bulbs. That's the only time that case had been opened since purchase. So I think it would be safe to say that the "professional" was inexperienced at best, or possibly even careless or lazy -- i.e. either lost screws or couldn't be bothered to replace what she had removed, perhaps thinking no one would be the wiser. Well, a year and a half later, I am now repairing phones and found the unprofessional workmanship.
LESSON? If you're not doing your own work, due diligence is needed to ensure that whoever is doing the work will provide you with good workmanship. And if YOU are that professional, make sure you don't leave a client with shoddy workmanship, thinking that once the device is closed up, they'll never know. Replace lost screws. Admit errors and replace parts at your own cost. That's how you win repeat business.
Last, since unknowns and surprises are inevitable when repairing devices it doesn't hurt to gradually build up a collection of spare parts, purchase a set up extra screws for various devices, have extra adhesive strips on hand (in case you sneeze at the wrong time and get one stuck to itself) -- well, you get the idea.