Oh I just hate it when I do this (which fortunately isn't too often). I put the first border on the purple stars and apparently pulled somehow because the border was all wobbly. I hung it up on my bifold doors (as I usually do) and it was not at all flat. Alas!
So I have 3 of the 4 borders off and finishing up the last one. Then to re-measure and start again. Sigh.