"Omg I thought his lil heel was a nipple," one fan pointed out.
"Totally thought it was a nipple! Then realised it was foot...." another said.
"Hahaha I zoomed. Not gonna lie," one fan wrote.
While one admitted she "wouldn't have even noticed" the apparent nip slip, but since it had been pointed out they "can't unsee it".
Many others admitted to having to "look twice" at the tender snap because of the embarrassing optical illusion.
Despite originally writing a sweet message to her little boy about how his birth had made life "content", Abby later amended the caption to address the many comments.
"Ps that's his foot, not a nipple," she wrote, along with a laughing emoji.
Her full message to Mason talked about the amazing connection the pair had formed in the short time since his arrival.
"It's amazing what one little boy can do. You've made me relax. You've made me feel content. You've made me enjoy this time again and most importantly, you've made my heart feel so complete little man," she wrote.
Adding: "Luckiest Mummy to have my 3 beautiful kids who share the strongest connection already."
Mason is the first baby for Abby and her boyfriend Leighroy Wellington. She already shares two older children, Arlo and Milla, with her ex-partner Jake Stringer.
She split with the Essendon player signed in mid 2016 and has become vocal in the "mummy blogger" community — sharing details of her life with the kids and running workshops for women.
She regularly shares brutally honest posts about pregnancy and motherhood, telling her 109,000 followers her boobs were "veiny" when Mason was born as he was desperate to feed immediately.
In another post she described how hard breastfeeding was, describing her first few days as "rough".
