Sandakan is a district in the east coast of Sabah. It is also the second largest town in the state after Kota Kinabalu.
Former Sabah chief minister Yong Teck Lee in a tweet, accompanied by a graphic of the blunder, said:
“New Malaysia’s RTM1. Sarawak flag. What a joke!”
Others pointed out that RTM1 did not know the difference between the two East Malaysian states.
One Facebook user, Joseph Wilfred Lakai, wondered how the station got it wrong.
“So Sandakan can move to Sarawak?”
One Gerache Grace labelled RTM “stupid” and questioned the ability of its editors.
Many social media users tagged the communications and multimedia commission ministry on the blunder.
The Sandakan seat fell vacant on the death of the MP, Stephen Wong Tien Fatt, last month.
Wong, who was Sabah DAP chairman and state health minister, had been MP for two terms.
Nominations for the by-election will be held tomorrow morning (Saturday), with early voting on May 7 and polling on May 11.