"Because a candidate with a mediocre background yet a strong showing on the whiteboard will get the job."

Is there evidence that this happens a lot? Does this happen at Google, where whiteboard programming during interviews is common?

"The problem is the over reliance on the whiteboard signal, despite a history of contradictory evidence (either positive or negative). If you are going to use typical whiteboard problems as a signal, it is really really hard to not heavily weigh the results, especially when sour."

What history of contradictory evidence? Do you have any suggestions for other signals that have proven effective in your experience?

I have very, very little experience as an interviewer and no experience dealing with and evaluating the success of new hires in the long term, so I definitely don't claim to have the answers here.