It is easy to leave the battery cooking on the charger over the weekend or for days until it is needed. The battery ends up right back where you started, overcharged. If you use only half of a battery's rated capacity but give it a full overnight charge you can overcharge it and send the cells into voltage depression. Most of these overcharged batteries will appear to have the dreaded memory set problem. Nickel-cadmium memory, or a condition where the battery gets partly used and then recharged many times until it only has that capacity, is actually quite rare.
Overcharged batteries are much more common.
The solution to overcharging or memory problems is to use the full battery capacity before recharging. If necessary, have an extra charged battery available. This will allow you to fully use the battery capacity before having to recharge it. Only charge the battery for the manufacturer's recommended time. If possible, use a charger designed to shut down when the battery is fully charged.