The wolf in the forest
"Be careful in the forest -- there are wolves in there."
"Nonsense. I go through the forest every day and I have never been attacked."
As fallacies go, this one's pretty clear. Something does not have to occur to everyone every time in order to be a danger. Knowing that there's a wolf in the forest, the second traveller should alter his behaviour to minimise the risk. Carry a weapon, sleep next to a fire, the usual stuff.
When it comes to language learning, though, people do tend to take the attitude of the second traveller. Point out any of the potential pitfalls in a language learning strategy, and the other person will usually accuse you of talking out of your hat, and point out that he learned OK that way, or that some of his students did.
But just as one man emerging safe and sound from the forest doesn't disprove the presence of wolves, the success of one or two language learners doesn't demonstrate a lack of potential pitfalls in the methodology they employed.
People tend to have a hard time accepting this, though. I point out a pitfall, and they declare it isn't a problem. "And I'm living proof." When I try to point out the fallacy, I'm often accused of disrespecting their experience. No no no. I respect and acknowledge their experience, but my point is that there is more in the world than one man can experience. Our capacity for reason allows us to go beyond our experience, and we should take full advantage of that. We should not limit ourselves to our own experience, and we certainly shouldn't limit others to it either. We need to reconcile our experiences with the knowledge of others, and thereby remove the pitfalls and reduce the risks before giving advice to others.