As I understand it, all informal fallacies are formally valid. The problem is that informal fallacies follow from weak or demonstrably false premises.

Is that true?

If so, it occurs to me that some arguments that could fall under the heading of "informal fallacy" could actually be trustworthy arguments.

Example of trustworthy "informal fallacy:" (Slippery Slope Type)

If a toddler is toddling toward the busy highway now, he will continue toddling in that direction until he toddles into traffic.

The toddler is toddling toward the busy highway now.

Therefore, he will continue toddling in that direction until he toddles into traffic.

Obviously, the person who does not act on this conclusion because it is technically an informal fallacy is not being reasonable.

Wouldn't every type of informal fallacy yield some examples of trustworthy conclusions?

First of all, I really like this comic

It seems to me that there has been a trend on reddit of pointing out logical fallacies in cases when the arguments are valid. There are often situations where either the definition of a logical fallacy doesn't really apply, or the "fallacy" isn't really wrong at all. Some logical fallacies are obvious, like strawman - but a strawman is essentially a lie, and we all know not to lie. Here are some specific cases of fallacies which I think are broadly misapplied:

1. Argumentum ad hominem - while appealing to the opponent's characteristics is usually a weak argument in my opinion, I don't think it's right to dismiss this kind of argument as "fallacy". For example, when somebody has an incentive to promote a specific viewpoint, it is likely that he omits important evidence to the contrary, and therefore since we don't have infinite time to investigate the case thoroughly, disregarding that person's argument might be a useful heuristic in determining the truth. A more obvious case is dismissing the claims of a known liar. I think ad hominem arguments should be judged on a case by case basis instead of being dismissed altogether. Tu quoque is a more specific case of ad hominem, which might be useful for the same reason.

2. Appeal to authority, on the other hand, is kind of a reverse ad hominem, when we use a person's credentials to argue for their position. The website yourlogicalfallacyis.com says that appeals to authority are wrong when we say "it must be true because an authority says that", but I don't think people use deductive arguments often. We usually say "X is likely because Y" instead of "X is certain because Y", therefore, ad hominems and appeals to authority are fair game.

3. Argumentum ad populum. Again, it might be a useful heuristic. For example, the reasoning might go like that "the vast majority of people think Lincoln was a good president, so there must be some good reasons for it, therefore it's likely to be true". An even better case is when we accept a claim based on the fact that the vast majority of scientists believe it. However, ad populum is not necessarily a strong argument, so there might as well be an argument which outweighs it.

4. Slippery slope argument. It's a type of argument from consequences, which is completely valid if the slippery slope can be expected to occur. Similar to ad hominem, slipepry slope arguments should be judged on a case by case basis. For example, Eugene Volokh wrote a good article on why the claim "gun registration may lead to gun confiscation" might be true.

5. No True Scotsman. This is a specific case of redefining a meaning of a word and pretending it didn't change. However, it's not wrong if I assumed the definition from the beginning. Moreover, plenty of times simple claims like "true Republican must oppose Obamacare" are dismissed as "No True Scotsman". While my definition of a "true Republican" might be peculiar and might lead to disagreements, it's obviously not a logical fallacy.

In short, I think people often confuse deductive arguments with inductive ones. Am I right in this critique?