Police say that staff at the Bellflower on Parkside Lane, became suspicious after a pair attempted to pay for a soft drink using the note on Monday, June 18 at around 3.30pm.
But eagle-eyed staff at the pub spotted the forgery and challenged the customers who then made-off in a dark blue car.
A spokesman for Garstang police said: "Thankfully, vigilant staff at the pub spotted the forgery as they are quite switched on to fake notes.
"But we wanted to warn people in Garstang to look out for these notes.
"This is the second attempt in three weeks where people have tried to use a fake Â£50 note at this bar.
"But it should be noted that the people in the first incident, were not the same people who were involved in the second attempt.
"They are good forgeries and are quite hard to spot."
Police are now appealing for members of the public to help them identify two men who they believe may be able to help them with their investigation.
Anyone with information should contact PC 2733 Anderson at Garstang Police or call 101.