Oh dear, I'm with you on the mistakes. At least we can admit them and giggle about it.Take you time and cut it one more time.
I messed up marking lines for quilting a piece on the embroidery machine last week. It was a sampler piece with a 2 inch border. I marked one side at 2 1/2 inches. The class instructor says she never uses those type of regular OR half inch sided rulers due to the mistakes. Guess I should retire that one to ruler folding only.
PS: I've made the "same" mistake 3 times!