I have 3 books that were sent in 3x each for repeated Mechanical Errors - tbh I gave up after round 3, as there was evidence of damage to one of the actual books, and the 3rd round of slabs had the least glaring of the Mechanical Errors (mostly large scuffs, a couple had clearly visible scratches, debris, etc.)
Fwiw, I have had only one book come back in the last 6 or so weeks (out of ~10? books) that went back for a Mechanical Error - and it was pretty minor (still clearly visible, but not as glaring as some of the earlier ones), so I definitely think there's been some recent improvement to the processes.