George Martin may have been the surrogate music expert behind The Beatles, but he knew when some sounds weren't worth the risk.

John Lennon and Paul McCartney were each other’s favorite audience. That was plainly clear as the besotted Beatles bantered, bickered and obsessed over the 23 years they were friends and rivals.

The Beatles constantly blazed trails and set examples in rock and roll, and here are five occasions where other artists ...