W8,
Detonation, auto-ignition, bad maintenance. Every pilot should inform himself about the proper leaning procedures for his engine. Even the POH is no always to be trusted since it was first written by engineers (mostly correct) then proofread by the lawyers (....) and then 'adjusted' by the marketing dept.
No proper engineer would recommend operating at 50°ROP at high power, but it suits the marketing dept just fine, giving a few more knots (with no regard to engine life).