Well, as we all know Shanks has a scar on his left eye which is given to him by Marshall D. Teach aka Blackbeard; the scar was absent on Shanks when he was part of Roger crew.
When Shanks visited Luffy at Foosha Village he had scar. So he received the scar from Teach during the period between Roger’s execution and the visit to the Foosha Village.
Here’s my theory. I’m going to start with the devil fruit which Shanks had (Gomu Gomu no Mi/Hito Hito no Mi, Model:Nika) and the devil fruit which Teach wanted (Yami Yami no Mi). Both the devil fruits seem to look similar for color and design. So it’s safe to assume that one can get confused on recognizing them. And if you are as desperate as Teach well you will.
Now, when Shanks entered Whitebeard’s ship Moby Dick, the reactions from various crew members were like they were meeting Shanks as Yonko for the 1st time, those who fainted on his arrival. Even Shanks’ conversation with Marco “so, you are 1st Division Commander Phoenix Marco” states that Shanks and Whitebeard never clashed as “Yonkos”.
At some point in his life, Shanks encountered Marshall D. Teach again, then a member of Whitebeard’s crew. What exactly happened is unknown, but Blackbeard left Shanks with the 3 scars across his left eye. The exact date of that event is unspecified. However, there are signs indicating that those scars were made near the time of Roger’s execution: while images of a younger Shanks depict him with his face unscathed, in two flashbacks (in Chapter 434 and Chapter 0), he has his face hidden by his hair and his hat.
As we know, 12 years after the Roger Pirates’ disbandment, Shanks’ crew stole the Gomu Gomu no Mi that was being guarded on a government ship by then CP9 agent Who’s-Who. In that same year, Shanks arrived on Dawn Island, where he met Luffy.
Teach was already after Gomu Gomu no Mi mistaking it for Yami Yami no Mi, which he has seen in the book of devil fruits. So when Teach arrived, he found out that the fruit was already stolen by Shanks. So he planned to get the devil fruit from Shanks and we know how evil planner Teach is.
*Theory by blaze
