Devices that use a multi-cell NiCD or NiMH battery pack often will operate until some minimum voltage output from the battery pack is no longer available (and the device turns itself off). If that minimum voltage happens to allow ANY of the cells within the pack (especially if those cells are not perfectly matched in capacity/voltage characteristics) to drop below the minimum allowed voltage per cell, then that cell is internally stressed and is aged more rapidly than the others in the pack

Multicell (ie most) packs do suffer from capacity mismatch purely due to manufacturing process tolerances, and this can cause problems for both charging and discharging.
The discharging problem is because the weak cell can go flat whilst the other cells are still capable of driving current through it. This screws up the cell chemistry and damages that cell, thus creating a viscious spiral. NiMH cells are more susceptible to this damage than NiCd.
The charging problem is because the only way to tell when a cell is fully charged is to monitor it's voltage throughout the charge cycle - it will increase steadily until charged when it will peak and drop off slightly. At that point any extra energy supplied to the cell is disippated as heat and damages the cell as it does so. With 6 cells of varying capacities, that peak is going to happen at different times. This means that the voltage drop of the weakest cell may be masked by the steadily increasing voltage of the other cells, and the weakest cell may thus be overcharged. The worst case scenario is that the charger misses every peak except the last cell, and every other cell is overcharged.
These two issues also have the side effect of limiting the pack's usable capacity to that of the weakest cell. For most applications this isn't a problem - we don't really care whether the Karmas battery lasts 18hours or 18 hours and 20 minutes. But in the world of competitive RC car racing every coulomb counts, people pay ludicrous money for 'matched packs', where each cell has been individually measured, and the packs have been constructed out of cells with very closely matched capacities. Obviously the better the numbers, the more expensive the pack.