I've found enough "surprises" when altering that I now prefer to only work on ones I've made myself! Can't quite make it an absolute rule, but I WISH I could.
One time I had an apron almost apart to enlarge a kilt before I discovered that a 6"x8" piece had been cut out of it ... looked like just about the right size for a new set of buckle tabs.
And a few years ago I was altering a number of Burnett's & Struth kilts for a pipeband and they were all made with the wrong side of the tartan out!

Ron.