Can buy items even though not enough gold?

I just played a game of support morgana 10/4/2016 at around 11 PM CST. In the game I had 2k gold after selling sightstone(it was late game). I then noticed I could buy deathcap even though the price was 2550 gold. My gold said 2000 and I could still buy deathcap. This isn't the first time this has happened to me, it has happened 2 times previously(in howling abyss), but I never thought anything of it til I could leave shop and deathcap still stayed in my inventory. I have no clue how this happens, but if you give me like 50 games I'm sure I can replicate the bug again. Either gold isn't displaying properly or the item price is bugged for me. Lost the game anyways. Willing to do anything to help solve this issue to prevent people from cheating in ranked.