Batteries charge, hold charge, and discharge at different rates. Surprisingly, this is true even if the exact same cells are used every time, in series, in a device.
Over multiple charge / discharge cycles, one cell can easily end up fully charged, while another is only at 50% charge. If the cells are in series, the flashlight will still go dead after the 50% charged cell is empty, which is annoying.
If you keep trying to use the light with cells in series, and one is substantially lower, the cell can go into "reversal" and be ruined. For NiCd and NiMH, this is just annoying, but not dangerous.
If the cell is Li ion or LiPO, and it goes below its minimum voltage spec (even if not gone into reversal) it can be internally damaged. 2 -4 charge cycles later, it can fail catastrophically and the fumes are really bad for you. This is why you charge Li based cells in a steel bucket with sand - outside. They even sell "charging bags" for them.
If the cell actually goes into reversal, it nearly always will fail catastrophically. There are some other ways things go wrong as well, but maybe that is enough to make the point.
In order to reduce the probability of this happening, devices (like your phone, computers, etc) which use them have a number of safety features, for example:
- Temperature monitoring
- individual cell discharge voltage monitoring
- individual cell charge voltage monitoring
- Max discharge rate fuses
- over discharge voltage trip sensors (per cell)
- Ability to charge / slightly discharge individual cells in a pack during the charge cycle
This is why we can all safely use Li ion and LiPo cells in our daily lives, they are "nearly fully protected" and there are companies that make dedicated chip sets for this specific purpose.