It's a small thing perhaps that so many flashlights become unreliable with use, but it's been a pet peeve of mine for quite some time.
Many flashlights depend on the tip of the bulb to make contact with the positive end of the upper flashlight cell as sketched below on the left. This works okay for a while, but eventually, the metal of the bulb's tip deforms by flattening out while the positive tip of the flashlight cell also deforms by becoming concave as sketched below on the right.
Then the flashlight doesn't stay lit as I hold it. I have to shake a little it to get light or I have to twist the light bezel back and forth a little bit.
I'm sure you know the drill.
When this happens, I can put a wad of aluminum foil between the bulb tip and the positive tip of the flashlight cell to make things okay again, but I really resent that I need to do that.
These days therefore, if I buy a flashlight, I look for one that has the bulb encased a protective housing with a brass tab for contacting the upper flashlight cell. Nothing deforms.
Of course, as you might guess, one of those "better" flashlights that I bought now has an intermittent sliding switch.
Edsel Murphy lives!