One of my favorite fallacies is this one:

"Contrails don't persist; 'chem-trails' do."

This is used on a regular basis as "evidence" or "proof" of "chem-trails", but just a teensy bit of critical thinking debunks it right away.

Even the most rabid "chem-trail" believer will admit that "regular' contrails are just frozen or condensed water vapor from the aircraft engines, just like normal clouds.

And since
(a) normal contrails can't persist; and
(b) normal contrails are the same makeup as normal clouds; then
(c) they will behave the same way, which means that normal clouds can't persist, either.

Except, of course, that they do!

Normal clouds can persist for hours and days on end, which means that
(a) there is something like "chem-trails" even in "normal" contrails to "allow" them to disappear, which, in turn, means that
(b) they're not normal contrails and
(c) you now can no longer discern "normal" contrails from "chem-trails"; thus
(d) the argument falls apart.