I have my interview in a few weeks with one of the MBB, and just realized, looking through my cover letter again, that I made a big mistake on the cover letter I applied with for the position. I have, in one sentence, said that I did an internship last summer, while I actually did it two summers ago (summer of 2015). This is because in another cover letter wrote the same, but then about a year ago. It is just this minor detail, but it really bugs me as I think I might come forward as not precautious enough with the things I write, and they might think I just copy paste my cover letters. Apart from that, it looks fine, it is just this minor detail which they will notice if they combine my CV and my cover letter.
What do I do? Do I contact HR and send them a new cover letter, or do I let it slide and hope they won't notice? I must also mention that I already had to resend them the cover letter because of a problem on the online application, which makes me hesitate to contact them again once again.
Thank you for your response!