A category mistake is when, for the ontology of any particular domain of discourse, an element of a set is unjustifiably excluded from the set.
An obvious example of this is the statement, "It is a time before time". Time cannot be before time because the extension of time includes the extension of every statement of something being before or after something else (i.e. every B-series statement) such that "a time before time" falsely excludes "time" from the extension of "time".
A less obvious example is the statement, "It is before time", however this statement has similar problems to the one before because it is to say, "It is at an earlier point in time than time." This falsely excludes a point in time from time, which is false by contradiction.