Most home jobs do not go wrong on the first day. They go wrong on day four, when someone opens a wall, finds the sill is soft, and says that is going to need one more thing. The estimate described the job you asked for. The new thing is not in it. If nobody writes it down, the house gets the repair and the paper still shows the old plan.
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A change order is the page that records new work. It names what is being added or removed, what it does to the schedule, and what it adds to the original number. It should be signed by the person who has authority to approve changes in your house, which is often you, and by the person who will do the work. A verbal agreement about a change is the same kind of promise as the one you made about keeping her at home: sincere, and easy to lose.
What to check before the new work starts
Ask the contractor how changes are handled before the job begins, and get the answer in the contract itself. Look for who can authorize a change on site, whether your signature is needed before the crew starts the new work, how long a price stays open once a problem is found, and whether the crew will pause the affected work until the paper is signed.
Read the words in the change order. Each one should say what is being done in plain terms, such as replacing a section of subfloor under the bathroom, not 'additional repair as needed.' If the description is vague, the price will probably be vague too.
Check what happens to the original scope. Does the new work replace something on the first page, or sit on top of it? If it replaces a line, that line should be crossed off on the paper. Otherwise you may pay for the same work twice without noticing.
What to keep
Keep every change order with the original contract, in one place. A change order that lives in someone's truck is not the same as one in your folder. When the job closes, the stack should show the first agreement and every change that followed, in order. That stack tells the next person what the house actually has behind its walls.
Ask for a copy of each signed change at the time it is signed. Then, before final payment, walk the house and check that what was changed matches what the paper says. If something does not match, bring it back to the contractor while there is still room to talk.
When a change touches safety
If a change involves gas lines, the electrical panel, or the structure holding up a room, ask the contractor which licensed trade will do that part and whether that trade signs its own paperwork. A change order can record that work was done. It cannot make the work sound. Whether a change needs a permit or an inspection is a question for your local building office and the contractor to settle together.
The paper is not there to make you suspicious of the people doing the work. It is there so that when the job grows, both of you still agree on what the house is getting.