The right pipe, the right leaf, the right humidity, and it has certainly happened to me on a number of occasions. So what? If it doesn't taste good, the smoke is over so far as I'm concerned. The amount of leaf that's left unburned is mostly insignificant, usually even less than I suspect. This white ash myth is not only an exaggeration, but boring one. So you have three particles of leaf left in among the white ash versus a little bed of thirty pieces of leaf. Only the psychiatrically anal retentive could notice or care. It's kind of a daydream to light up an advertisement, but otherwise, meaningless. If you always have this experience, more power to you (?).