I bet we all have seen this error from time to time. This usually happens when you create new managed properties and try to expose them on the search center, but it can also happen is cases when always. There is a check list:
1. Make sure that the managed property has been added to the managed properties and mapped to crawled properties
2. After adding managed properties start the crawl
After ensuring that the managed property has been created and populated with values, ensure that your web parts are referring to this managed property and the name is not misspelled. if you still get this error, it might be because the SharePoint Query components are not updated. Restart the IIS on the query servers, to resolve this error.
With FAST, there are also additional thing that can go wrong. In some cases you can see this error on OOTB Fast search center as well and in can be inconsistent if you have multiple query servers. This is indicative that the query servers got out of synch.
The Index Schema defined in SharePoint Central Administration at the Query SSA's FAST Search Administration section is stored in SQL Server, and deployed to FAST Search through a timer job. It's possible that not all steps of the deployment occur, leading to a newer schema in SharePoint / SQL than the FAST qrserver is using.
It is easier to restart QR components to get them back in synch than to find the one out of synch and re-synch it.
Here is what you can do to resolve it:
Open up the FAST Search for SharePoint powershell and run the following on each QR server.
(Only do the ones that exist from the list from nctrl status)
nctrl stop qrserver qrproxy search-1
nctrl start qrserver qrproxy search-1