First, I'll address this, as it's important for the rest to make sense

> Government intervention doesn't have to be unforeseen. Good regulations are announced in advance, and businesses are informed during the legislation process and given guidelines to adapt and comply.

You seem to not understand what is meant by "unforeseen negative consequences". These are results of a policy which weren't foreseen by the proponents of that policy. Often, policies meant to have positive_effect_A have negative_effect_B, C, D, etc. Sometimes they entirely fail to have positive_effect_A and even have the opposite of that effect. Rent control is one popular example (meant to reduce the cost of housing for the poor, but in reality raises the cost of housing by reducing the supply). Sometimes nobody sees these issues coming, but often opponents of the policy point out these problems but the policies are pushed through anyways, due to emotional support.

Now for the rest

> This is a blanket statement that doesn't really make sense.

Yeah it is a little too absolute, I should have said "Market failures are rarely improved by government intervention, and often made worse".

> There is no way to correct externalities without regulation, right?

Common misconception. Often externalities are caused by government policy in the first place, so reversing that policy can fix it. Another option is increased liability tort law: allow individuals to sue for damages and you'll quickly see companies make changes.

In my opinion, one of the policies with the most negative consequences is the creation of "limited liability corporations". Of course when the owners and operators of a firm aren't liable for the actions of the firm, they don't have a vested interest in instituting internal policies to limit the firm's harm.

> Government intervention distorts the market, which is desirable in some cases.

Desirable to some, again it's subjective. Often distortion is created and used by rent-seekers at the expense of everyone else.

> There would be a market for paid assassinations, but we've made that market "worse" by outlawing them.

Assassinations are in an entirely different class of action, one which is not part of an ideal free market. A free market is one in which all interaction is purely voluntary. Violent crimes, fraud, theft, etc are actions in which the victim did not consent.

Actions which violate consent should be illegal, and the justice system should be focused on taking every possible action to make the victims whole. This is different from how the justice system currently works, which is focused on punishment of criminals rather than reparations to victims.

Market failures result from actions which do not violate consent but do still cause harm to a third party. Air pollution is one strong example, and regulations for that (and other forms of pollution) are the most justified regulations IMO.

People will also bring up "natural monopolies" and "oligopolies" when talking about market failures. However, there is very little evidence of a hostile monopoly or oligopoly ever existing without government assistance, while there are countless examples of cartels failing and diseconomies of scale destroying large organizations.

> I feel like it's impossible to argue that the market is always "right".

Because "right" is subjective, that is impossible. There are things like so-called "price gouging" in disaster situations which many people perceive as wrong, but in actuality are a necessary signalling behavior of the market in order to allocate more resources to those areas. Banning such things only results in shortages which _do have_ measurable negative consequences.

We can look at metrics like cost-of-living adjusted standard of living, inequality, innovation, economic growth, etc and all of those correlate strongly with how free an economy is.