The PID Rug Bump Challenge is the ability for a small robot (with 35mm radius wheels) to confront the edge of a thick Kilim rug (17mm thick) with one wheel without being deterred from its path, i.e., recovering from that bump whilst maintaining its original heading.
The "ideal" behaviour would be follows, quoting David Anderson of the DPRG:
Tuning the PID Controllers so that they respond in kind is the challenge.