My buddies and I were running a game with an academy in it and whenever we would RP well the DM would have us roll a d20 to see how well we did "training" on that day. Whenever we would do well he would put a stat boost item into our equipment then remove it whenever a week had past, reverting us back to our original stats. This worked well up until we leveled up with the charactermancer, while we still had the stat boosts. The charactermancer would take the stat boost and add it onto our character during the level up and we would come out of the level up with the stat boost permanently on the character. For example, my character had a +2 to his strength, taking it from a 16 to 18, and I still had it when I leveled up, which caused the charactermancer to take my 16 original strength to 18 original and now after the level up I had a 20 temp strength. Since my buddies and I realized it worked when we added nameless items that gave temporary stat boosts, we wanted to see if it worked with things from the PHB and it turned out to also permanently increased the stats with items such as The Belt of Dwarvenkind.
