Problem swap: the point s is on an edge

Hi all, I received the following message while running my code:

Problem swap: the point s is on an edge => swap 0 [480689017::1911157094,1911153325,1911153324,{480688784 4} {480696710 0} {480696711 0} ]

Can anyone tell me what this means? Many thanks!

Someone may know off hand but if it is obscure it may help to post the code generating that output. If it is from FreeFem, you could grep the source code for parts of the phrase
likely to be present as a literal.