Hmm you confuse me... You say excess base/metal/carbonate which
suggests you are talking about obtaining a soluble salt that does
not have potassium, sodium or ammonium ions, but you also said
precipitation.... Anyway, I shall answer to the former because it
is most likely....
Key source of experimental error... You may not have stirred
the mixture well enough or heated it for sufficient time to
increase reaction rate. As such the solution may still contain some
amount of acid and not all of it may have reacted. Therefore there
may be some amount of contamination.
That's the best I could come up with. >_>