This is the honest lesson. Claude sometimes states wrong things with total confidence. Two simple habits protect you, and they take ten seconds each.
Confident guessing
Ask Claude something it doesn't know and it won't always say "I don't know." It's built to produce an answer, so when the facts run out, it can fill the gap with something that sounds right. The tech world calls this hallucination. Call it what it looks like: confident guessing.
It's not lying. Remember lesson 1: the new employee. Ask them your store's hours on day one and a nervous new hire might just guess, because you asked and they wanted to answer. Claude does the same, and it guesses smoothly.
This is not a reason to quit. It's a reason to build two habits.
Habit 1: give it permission to say "I don't know"
One sentence, added to any prompt where facts matter, changes Claude's behavior noticeably:
The safety line
If you're not sure about something, say so instead of guessing.
Here it is in a real prompt. Watch what it does:
Copy-paste prompt
What are the exact permit requirements for replacing a residential water heater in Lee County, Florida? If you're not certain of current local requirements, say so and tell me who to call.
Without that second sentence you might get a tidy, plausible list. With it, Claude tells you what it's confident about, what it isn't, and points you to the county. That's a far more useful answer.
Habit 2: verify what matters
Before anything Claude wrote leaves your hands, check the load-bearing facts: prices, dates, names, phone numbers, legal and code claims. Not every word. Just the parts where being wrong costs you money or trust. Ten seconds against your own records or the primary source.