I own a prelit,12-ft Christmas tree. Incandescent bulbs, about 15 years old. Changing out some bulbs today before I totally take it down. Find a spot with a cascade failure. I turn it off. Replace the bulbs. When I turn it back on, bulbs still not lit. Used my LightKeeper Pro. Lights on after a click or 2. My question is, the lights from that strand that have been replaced are now blindingly bright. I turned it off to avoid another cascade. I suppose it’s possible that I missed a few burnt out bulbs since the tree mostly still has decorations on it (and I really need to dissemble the tree to really get in there). What else could be going on? (My first thought was that the replacement bulbs were mislabeled, but I’ve used them on a few single bulb replacements here and there, and they appear to be the correct ones). Thanks in advance for any advice!

If the replacement bulbs are correct, most likely you've missed multiple burned out bulbs (easy enough to do since they can be fairly well hidden in a prelit tree when off).

If I have a set that I know has multiple dead bulbs, and want to avoid cascade failure while fixing it I either rig it to be in series with another set(that wouldn't work for a prelit tree though), or put it on a dimmer and turn the brightness all the way down to start (then slowly turn up enough to keep it no more than normal brightness)