Recently we were sporadically getting the following error on our site:

An error has occurred performing this search. Please try your search again

Exception:
message - There was a problem executing the CFSearch tag with the following collections.
detail - Collection (status code): programmes (-2)
type - SearchEngine

From diggin around there was not alot of helpful info on what exactly a -2 error is... aparantely its an error code for any error that does not specificaly have its own error code... incredibly helpful.

In our case we narrowed it down to race conditions on our verity collections. We had cflock tags around all verity access scripts however as we had multiple front end server accessing the same collection there was no provision to stop each server from accessing the collection at the same time as another. The was resolved by have a collection held on each of the load balancers.

 So take a look and make sure your code has cflocks around any access to the collection and in a that only one server can access the collection at a time.