He used to sneak into clubs as he did not meet the age limit of 18 years, but Mark (not his real name), now 20, still does it for the thrill of it, and to avoid paying entry fees of $25 to $35.
Last month, he "smuggled" himself into a nightclub by heading to its smoking area, befriending other partygoers, and following them back in through the back door without paying.
Mark said he has entered, or helped others enter, clubs without paying around 20 times since 2015, and has used a friend's identification to forge entry before as well.
"I used to do it because I was underage, but why pay when you can go in for free?" said Mark, a research and development associate earning about $2,000 a month.
Misuse of identification, changing birth dates on identity cards and faking entry stamps are the most common methods used to enter clubs without paying, said operators.
But some means have become more sophisticated and organised, as young people have more ready access to substances such as UV inks - used for entry stamps - and to services online that allow them to fabricate stamps, added the operators.
Last Wednesday, police said 10 teens were being investigated for three cases of forgery.
In one case, stamps of an outlet were believed to have been illegally made, sold and used. In the other cases, genuine stamps were said to have been duplicated and transferred among suspects by skin contact.
Mr Daniel Cheng, managing director of Get Juiced, one of the outlets involved in the incident, said forged entry passes are often used to bypass age requirement checks, and to avoid paying the cover charge. He said the club in Clarke Quay sees four to five patrons trying to sneak in on average daily.
Besides transferring the stamp to another's hand or wrist while the ink is wet, some enter in a big group when the club is busy and hope that security misses checking whether they have a stamp, he said. They then approach staff in the outlet to request for a replacement stamp.
While some may create a fake stamp, this is less common, he said.
A spokesman for Zouk, the other club involved in the recent incident, said those who try to sneak in are usually underage students between 15 and 17 years old.
Like some other operators interviewed, she did not say how often Zouk catches such patrons, or how many cases it sees in a month.
While patrons' methods have not changed much, she added, "the difference is that now, with the accessibility of the Internet, these methods of forging entry are made public and can be disseminated easily".
f.Club, another outlet in Clarke Quay, saw a "significant increase" in cases of forged entry this year, said managing director Volkan Gumus. "There has also been an increase in cases involving the misuse of IDs, and fake IDs."
Methods have become "more professional and organised", he said. "From transferring chops from skin to skin, people have started producing their own 'fake' stamps and matching inks."
To nab such patrons, security is extra vigilant at re-entry points, said Mr Cheng.
A spokesman for Canvas, a club in Upper Circular Road, said it uses stamps that can be recognised in only one orientation, meaning that transferring entries by skin contact will change the design.
It is an offence under the National Registration Act to use another person's identity card and pass off as them, said lawyer Amolat Singh. If found guilty of forgery for the purpose of cheating, an offender may be fined and jailed for up to 10 years.
"If they were to photocopy someone else's IC, that would be forgery," he said, adding that depending on the severity of cases, young offenders tend to get warnings or be put on probation for such offences.
But asked if he feared getting into trouble, Mark said: "If one is willing to take the risk, he or she should be ready to face the consequences if caught."
