The memory effect was in the old NiCad batteries. Whoever is feeding you this information needs to catch up on batteries you see in use today.
Having a mobile phone is great when you need to talk on the go, but if you have a faulty battery, then you have to carry charger along with you throughout the day.
Many things can go wrong with a phone's battery, from the so-called "memory effect" to plain-old worn out batteries. Some problem can be corrected, but unfortunately, most of the batteries need to be replaced.
Nowadays overcharge is a more common problem in modern cell phones. When you overcharge a battery, it means that you leave the phone on the charger after it has reached 100 percent.
While leaving a battery on a charging station for a few hours after the charge has completed will not impact the battery too negatively, if you leave the battery on the charger for more than 24 hours you can overheat and permanently damage the battery.