If you’re pulled over by the police and they suspect you’ve been drinking, they’re probably going to administer a breathalyzer test. A staple in every officer’s car, the machines may not be as reliable as you would think.

Two states in the last year have thrown out the results of over 30,000 breathalyzer tests, according to an investigation by the New York Times. The readout from a breathalyzer can prove less than ironclad, and courts ditched the tests because of problems ranging from operator error to mechanical malfunction. If you’re looking at a driving under the influence charge, a high blood alcohol reading might not directly translate to a guilty verdict.

Questionable calibration

Georgia has strict rules that breathalyzers need to follow. The state must approve the model, and requires that it undergo calibration by a designated professional. Anything that falls outside these bounds could render a test inadmissible.

Missing maintenance

Like any machinery, a breathalyzer that isn’t maintained properly may not work correctly. Run-down readers may spit out inaccurate readings, no matter how often the police calibrate them. Altered parts can also lead to missed readings, all of which may add up to unreliable evidence.

Improper influence

While the breathalyzer reads alcohol in your system, it may not be able to tell where the alcohol is coming from. Breath mints and hand sanitizer may have traces of alcohol and can affect the outcome of your test.

Understanding where your chemical test may come up short can be crucial to mounting a defense. Errors in testing could mean the state has to come up with another fountain of evidence, which may be hard to find after the fact.