The man behind the record rise in oil prices to $100 a barrel was a lone trader, seeking bragging rights and a minute of fame, market watchers say.
Open outcry trading is being replaced by electronic methods
A single trader bid up the price by buying a modest lot and then sold it immediately at a loss, they claim.
The New York Mercantile Exchange said that US crude oil futures traded just once in triple figures on Wednesday.
Some analysts questioned the validity of the trade, though their concerns faded as oil set a record on Thursday.
New York light sweet crude climbed to a new high of $100.05 a barrel on Thursday.
On Wednesday, one floor trader bought 1,000 barrels, the smallest amount permitted, and sold it immediately for $99.40 at a $600 loss, said Stephen Schork, a former floor trader on the New York Mercantile Exchange (Nymex) and the editor of an oil market newsletter.
"They absolutely overpaid," he told Radio Four's Today Programme.
"He paid $600 for the right to tell his grandchildren that he was the first in the world to buy $100 oil."
Most trading in energy futures has shifted away from the trading floor and takes place on electronic platforms.
The Nymex, along with the Chicago Mercantile Exchange is one of the last bastions of "open outcry", where traders use frantic hand signals to trade securities.
In London, open outcry trading still takes place on the London Metal Exchange, where aluminium, copper and zinc are traded.
Electronic trading has replaced open outcry in most financial markets because it is seen as being faster and more efficient.
Supporters also claim that it is harder to manipulate the market when trades are executed electronically.
The dwindling liquidity on the Nymex trading floor has led to considerable speculation that the exchange will soon shut down the trading floor to cut costs.