Trump, whose insurgent election victory was built on his willingness to disrupt political convention, was supposed to cross his arms and join hands with the men on either side of him.
But as others clutched the hands of the person to their left and their right, Trump used both of his hands to clasp the extended fingers of Vietnam's Prime Minister Nguyen Xuan Phuc.
The doubling up left summit host Philippine President Rodrigo Duterte with a spare hand -- and broke the line into two parts.
The handshake chain is a set piece of gatherings of the 10-member Association of Southeast Asian Nations (ASEAN), who are meeting with leaders from China, the US, Russia, New Zealand, Australia, India, Canada, Japan and South Korea.
Trump is on the final leg of a tour of Asia that has taken in Japan, South Korea, China and Vietnam.
The famously bullish former businessman has largely avoided the kind of protocol pitfalls that can ensnare even the most careful traveller in this often formal part of the world.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.