Is the MLS Always Reliable?
The short answer is NO. Per the MLS, the "MLS is deemed reliable but not guaranteed" meaning the listing agent does his/her best to put accurate info into the MLS, but there can be (and very often are) errors in the MLS listing so it is important to do your due diligence to make sure the information is correct. Pulling plat maps to see accurate lot size, asking your home insurance agent to make sure the property isn't in a flood zone, hiring an inspector to verify the property has a humidifier, garbage disposal, etc, and using an appraiser to check the total square footage are all ways you can double check that the information in the MLS is accurate. The MLS relies on the listing agent to put in accurate information, which often the listing agent has to get from the county public records (which are notoriously inaccurate) as well as the seller and, frankly, some sellers don't know every detail about their home or might know about items done by the previous owners. That's why it's important to double check anything in the MLS that is important to you.
Has MLS ever been wrong?
Yes, it's often wrong. I get this particular question a lot after closing, when buyers discover something in the MLS wasn't correct such as the fact that the property has engineered hardwood floors instead of real hardwood floors (this isn't something the inspector will check as he doesn't have access to the MLS) or that the property isn't in the school district it was advertised to be in. Of course this is alarming, but, unfortunately the buyer is on the hook for doing the proper homework to make sure everything they wanted or needed is correct as there's no recourse to "go after the sellers" after closing because of an error in the MLS. That's why at the bottom of every property in the MLS it says "The accuracy of all information, regardless of source, including but not limited to square footage and lot sizes, is deemed reliable but not guaranteed and should be personally verified through personal inspection by and/or with the appropriate professionals." While agents have to report knowledge of erroneous data to the MLS to be corrected as they are discovered, the burden of proof is on the buyer to verify their new home has what they expected it to have.