Hiring a contractor always comes with risk. No matter how many references you get or how reliable they might appear when giving you their proposal, there’s always the chance that they’ve overstated their skills, or that their personal life will intervene—or that they’ll rip you off.

Most contractor scams are pretty obvious, and once you know what to look for you can avoid the worst of them. But some contractor scams are pretty subtle—so subtle you might not even know they happened, and have an overall positive impression of the contractor—at least for a while. Here are four subtle ways your contractor can scam you, and how to protect yourself when you’re ready to start your next project.

Using less expensive materials without telling you

This one can be very hard to detect unless you know something about home repair or construction. Essentially, your contractor agrees to use high-end materials for your project—expensive paint, or stainless steel roofing nails, or pricey natural stone tile. But they actually use cheap paint, or galvanized nails, or ceramic tile that closely resembles the real stuff. They still charge you the same rate, so they make a huge premium on materials. You still get a completed job that superficially looks fine—and maybe you’re even happy with the work—but you got ripped off all the same.

There are two ways to defend against this subtle scam. You could purchase the materials yourself, ensuring that you get exactly what you want and what the contractor bid on. Or you can ask to see all receipts and inspect all the materials before they’re used to make sure you’re getting what you’re paying for.

Driving up the price little by little

Negotiating with a contractor can be stressful, and that sense of relief when you settle on a price, a timeline, and a strategy for your project can be profound. Maybe you pushed to use a lower-cost material or approach, or you feel like you did a good job being hard-nosed on the price.

And then the changeup comes: Your contractor can’t find the materials they agreed to use, and the more expensive stuff has to be used. Or heretofore hidden problems in your house inflate the costs once they start opening up walls and floors. Before you know it, you’re somehow paying the original, too-high quote you started negotiating against.

This scam is very tough to deal with because it’s certainly possible that a certain type of material isn’t available, or that hidden problems are complicating the work. You can try to locate your chosen materials yourself to see if they’re truly unavailable, and you could bring in another contractor for a second opinion regarding the uncovered problems and proceed from there.

Skipping important (but hard to see) steps

When a renovation or repair project is finished, you expect it to look finished—but that finish can also hide a multitude of sins, especially if it’s a small job that doesn’t require permits to be pulled. Things that your contractor can skip that won’t be obvious until it’s too late include:

Priming. If your painters didn’t bother to prime the walls, they may look fine immediately after they’ve finished, but will start to fade and have irregular sheen over time—or worse.

Insulation. Once drywall or trim around a window or door is up, it’s very difficult to tell if the correct insulation was used—or if insulation was placed at all.

Venting. Your contractor installed an exhaust fan, dryer vent, or oven exhaust—but just vented it into your walls, ceiling, or attic instead of to the outdoors.

The best way to defend against a contractor skipping whole parts of the job is to build in checks when negotiating the contract. If you can see that they primed the walls properly before they start painting, you can be confident that everything’s being done as agreed.

Misleading quotes or charging practices

Getting what sounds like a fair price from a contractor can feel like a well-earned victory. You did your research and you knew the questions to ask, and now you’re getting the work done within your budget.

And then come the reveals: A litany of reasons that the cost of the project is actually higher than you thought. This can come in several forms:

The price quoted only applied to certain finishes or styles, not the style you thought you were getting.

The price was for a specific aspect of the job—labor only, and not materials, or just for a part of the work that needs to be done.

The contract specifies that literally any change comes with a steep add-on charge.

The price you were quoted was actually a base charge, and almost every aspect of the work is actually a separate charge.

The contractor gave you an “estimate” of the costs instead of a “quote,” which gives them more wiggle room on final pricing.

These revelations are usually combined with an exasperated look of confusion, as if it was all made very clear to you and the contractor can’t understand why you’re confused. Your best defense against the reveals is to review any agreement with your contractor very carefully and ask for language that specifies the total overall cost of the job.

Not all scams are clumsy attempts at ripping you off—some are rather subtle, even unnoticeable. Keeping your guard up and simply asking questions at each stage of your project can help insulate you from trouble.