I grew up "knowing" that all Islamic art would contain an imperfection; mother's knee "knowing".
But I did search corroborative proof, as I quite like to think it would be true, and found very little to support it, particularly from Islamic Art sites.
I love the concept of deliberate imperfection. If your finished piece is perfect, but for a deliberate mistake, isn't it still perfect? And that debate just goes round and round.
If it was perfect but for your mistake, I think any God would be able to call you on your attempt to appease...
At the Gates, "hey Harry, that wallet with the error in the pictish border; saw that one ... "
H