I don't suppose you play SR, CS, or PG do you? If so, has this happened on other maps? Negatron's one of those items that we didn't touch for TT.

I will on occasion, but I have not ran into this on any of those maps.

I will mess around with some customs tonight and see if I can duplicate it, if so I will let you know in greater detail what led up to it.

Just to give you all the info I can remember (sorry if some of it is useless, I'm not sure what you would need or not). Both times were while playing Riot Squad Singed, it was on new TT about a week after it released and then a day or two after that. The first time it happened I had a RoA, Merc Treads, Glacial Shroud and a Negatron Cloak. We had just aced and killed Vilemaw when I recalled back to base. I already had abyssal selected in the shop from my last back, and for some reason I chose to double click it instead of using the buy tab. Now, I think i double clicked the recomended item, but i really can't say I am 100% sure about that and may have double clicked the item in the shop itself after having it selected.

I hope that helps. Thanks for looking into it.

I would guess he wasn't paying attention to his gold ammounts tbh, The game is here:
http://www.lolking.net/summoner/na/24631955#history
Edit: this is about the OP's game, not Violents game. quoted the wrong red post.
He has 9.2k gold worth of farm, and 7685 gold worth of items at end game.

Boots of Mobility: 1,000
Rod of Ages: 3035
Athene's UnHoly Grail: 2950
Chain Vest: 700g

Total minions killed: 109
Length of game: 25 minutes.
Gold earned from gametime: 1950 (13g per 10 sec)
Approx gold earned from minions: 3379
Kills: 2- 600g
Assists: 17 / 2250g

Total estimated: 8179g not including Alter captures; turret/inhib gold gain/oracles/potions (which would make up the missing 1k)

G from Minions done w/ average of 32g per minion due to game length (30g base +1g every 5 min.)

you only spent 7685 gold though...

RiotNome was asking me about a different problem, though if you know how to find that game please do. Maybe I was mistaken though.

Ahh I was referring to the OP's statement, not your game =X How long ago was your game?

I've had this happen one time. I finished up buying a phantom dancer and kept the gold. Saved me like 2k.

This was on SR quite a long time ago. Could never reproduce it.

Just want to add I had this happen the other day; Was buying either a Brutalizer or a Phage (think it was Phage) and I ended up not using the sword in the recipe. Freaked me out at first, because I thought I had just straight up bought another one when I didn't mean to, but in the end apparently I hadn't bought it. Was really weird- and this was on SR.v Hope it helps, Nome.

Just posting to say I had the same playing as Taric on SR. Had a ruby crystal, and then when I purchased a Zeke's I was left with the ruby crystal still. I suspect its something to do with having enough money to purchase the Zeke's strait up not including the ruby crystal.

Maybe the dude was just stoned or did his math incorrectly. Seems like a pretty common issue for the average LoL player