PSA: My nephew's toy car revealed a huge dip in the subfloor
Was doing a tear-out at my sister's place and her kid was rolling a toy car on the old floor. It kept veering to one side, which made me take a closer look. Sure enough, there was a solid inch drop over three feet. Saved us from a nasty callback by catching it before the new install.
Did you hear some contractors keep a marble in their tool bag for the same reason? I read it rolls on even the smallest slopes you might miss with a level. That toy car trick is basically the same idea, just bigger. Smart catch before laying anything new.