Long story short, my parent's toilet tank was leaking every so often. I drew a line on the inside of the toilet tank to see if water was leaking out of the tank. I also turned off the water so it would be easier to see any water level change.
In any case, I saw the level drop and replaced the flapper. I THINK I installed it correctly as I followed the directions on the package and when I checked again, it wasn't leaking water (followed my above steps, etc. etc.).
Now like 3 months later, they again say that same toilet tank is leaking water. I haven't had a chance to look at it a second time but I'm just wondering what else could be wrong to make the tank leak water again? Did I install something wrong? What might cause an even slower leak that takes longer than 10 minutes to show up?