Got my band as a Christmas gift and noticed a couple of weeks ago the strap started splitting in the same place as most, where the metal inside the strap ends causing a hard edge which the strap is pulled against when wearing it on one of the smaller settings.
My band is medium and I wear it on the left wrist with the screen on the inside. My wife had a band 1 which she had to send off twice for repair, once for the clasp (no problem with getting a replacement), she now has a small band 2 and it seems the metal is curved in more for the small wrist so doesn't appear as vulnerable to the issue mine has had.
I wear my band at night where it probably gets more pressure on the strap than during the day when I'm at my desk mostly. I feel this is probably when this damage occurred, hard to tell through as the crack is on the outside of the wrist and when it was small was hard to notice. I certainly didn't abuse the device to cause damage, I wore it as the manufacturer intended (to measure sleep).
I sent my band off for warranty repair stating that I felt it was a design fault but got it back today saying I had caused physical damage so it wasn't covered! The split started as a very small crack right along the hard metal edge, this quickly developed into a larger split the width of the strap which is when I sent it off.
I'm not happy. I'd be interested to know how others have got on when complaining. No MS Support online right now to complain to, I'll try in the morning. It was bought directly from MS Store UK so I guess I could complain to the store itself, or maybe as it was bought with credit card go to them for compensation!?