if you recall and buy an item, then undo, you lose all the health and mana you gained since you had bought the item. this actually killed me one time from a jinx rocket... lmao. it's been a bug for a while now....
I think it's on purpose. Making a mistake in purchasing stuff will give you some penalty simply by instantly decreasing some of your HP & mana to stall your time a lil more.
yea not a bug , like if you buy a 
for example you are healing faster than you normally would right? then you sell because none of the items you built use so it would save you seconds that would normalu be used to w8 for hp regain or mana then you get an unfair advantage for no cost . the reason it's for every purchace is proactive like if they made a champ that said the more MS you have the faster you regen or something
