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 it is between that period from Roger’s execution & visiting Foosha Village, he received the scar from Teach.
Here’s my theory.
I am going to start with devil fruit which Shanks had (gomu gomu no mi) & devil fruit which Teach wanted (yami yami no mi)
Above you can see , both the df’s have similarities (at least i think that ). Both seems to look similar due to that colour and design. So it is safe to assume that one can get confused on recognising the either devil fruit. And if you are as desperate as Teach well you will.
So my theory is based on that similarities of df.
Now ,when Shanks entered WB’s ship moby dick, the reaction from various crew members were like they were meeting Shanks as Yonko for 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 WB never clashed as “Yonkos”.
So how & when did Teach gave the scar to Shanks ??
I think that Shanks(before being a Yonko) had found a devil fruit Gomu Gomu no Mi from a certain pirate crew somewhere in grandline(mostly new world).
And Teach was already after Gomu Gomu no Mi mistaking it for Yami Yami no Mi, which he has seen in Df book. So when Teach arrived, he found that fruit was already stolen by Shanks. So he planned to get the Df from Shanks & we know how evil planner Teach is .
He joined the crew of Shanks(remember teach wasn’t a infamous pirate then, he always leaved in shadow of WB). He managed to infiltrate the Red Haired Pirates & waited for his opportunity, as soon he got his hands on the Df, was caught by Shanks & at the same time he realised that df wasn’t Yami Yami no Mi.
Now in order to flee he tried to fool Shanks in a meaningless talk, while attacked Shanks when his guard was down giving him a bloody scar on eye with his claw & managed to flee from there.
The reason Shanks scar still ached ,was he was done by the misleading personalities of Marshall D. Teach.
Thanks for reading ^^