Mamba user Kenneth Wallace was caught when he returned to the city to complain about a £40 drugs purchase.
The explanation was offered in court where he admitted having the class B drug on St Peter's Gate on November 9.
A six-month conditional discharge was imposed on Wallace, 52, of Walton Crescent, Carlton. He must pay a government surcharge of £20.
Stephen Flint, prosecuting, told the city's magistrates' court Wallace was spotted with another man at 4.10pm.
"They were acting suspiciously in a place where drug activity often happens," said Mr Flint.
Wallace was asked if he had any drugs and said "no". He was then searched and police officers found three bags of 'green vegetable matter' which turned out to be mamba.
Mr Flint added: "He said he bought the substance the previous day, paying £40 for it, and said it would last him for ten to 14 days.
"But he said he was not happy with the quality and was trying to find the person who sold it to him."
Nick Walsh, mitigating, told the court Wallace had used class A drugs - the most dangerous - "all his adult life".
At the time of the mamba offence, he was homeless and Mr Walsh went on: "It is a fairly miserable existence living on the streets."
Wallace had recently been sentenced by a crown court judge who had "taken a chance" on him. This enabled him to get support from a worker who specialises in people with drug problems.
He has been working well under this arrangement, added Mr Walsh.
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