Professional Liability Insurance

You provide Internet security for a major news company. One day, hackers break into the system and change the homepage banner headline to "Everyone Subscribing to This Is a Pretentious Moron." (Or something way worse, but we couldn't go there...and wow, did the Shmoop Finance team ever come up with some really great ones censored by the old blue-haired ladies sittin' around this table. They woulda been great. Really. But now you'll never know. Sad. Creative censure. But we move on.)

Subscriptions plummet. The news company blames you. They file a lawsuit and are likely to win a big judgment against you.
Luckily, you have professional liability insurance. It covers you for these kinds of things.

The product is often known more specifically as errors and omissions insurance (or E&O insurance). As you can guess from the name, it covers you in case a client is hurt by an error you made (like if you made a mistake in your code that allowed the hacker to get into a client's homepage). It also covers potential omissions on your part (like if you just flaked and forgot to install the newest security update).

A professional liability policy will usually cover the cost of defending against lawsuits (lawyers' fees, etc.), as well as covering potential judgements or settlements (up to a point, depending on the coverage). There are laws requiring these policies in some cases, particularly for lawyers and doctors.