A man mistook a stranger with tattooed legs to be a loanshark runner who had harassed his uncle and attacked him with a kitchen knife.

Sales engineer Ng Hwa Lee, 34, was jailed for seven months on Tuesday after admitting to causing grievous hurt to technician Sim Beng Hui, 22, at the ground floor lift lobby of Block 624A Woodlands Drive 52 on April 24, 2011.

A district court heard that Ng was drinking at an uncle's home at Block 623 that evening when he received a call that a loanshark was harassing another uncle of his. He then rushed over to his uncle's block. When he reached the lift lobby, he saw Mr Sim emerging from the lift with his girlfriend, carrying two motorcycle helmets on his arm.

He assumed that the technician was the loanshark's runner after noticing that the victim's legs were tattooed. He asked the victim aggressively "what he was looking at'', then blocked his way and punched him on the face. The victim whose spectacles flew off swung his right arm with the motorcycle helmets at Ng who took out a kitchen knife and slashed him on his left hand, causing the bones and tendons to be visible.

The victim ran up the slope of a multi-storey carpark with Ng chasing after him.

Ng later realised he had attacked the wrong person after his uncle told him that the loanshark had left "a long time ago''. He became scared and threw away the knife.

He was arrested on Dec 8 last year at a police roadblock. He could have been jailed for up to seven years or fined or caned or received any of these punishments.