diff --git a/lenskit_implicit/implicit.py b/lenskit_implicit/implicit.py
index 6cb5c26..31a5dfb 100644
--- a/lenskit_implicit/implicit.py
+++ b/lenskit_implicit/implicit.py
@@ -45,6 +45,8 @@ def fit(self, ratings, **kwargs):
         matrix, users, items = sparse_ratings(ratings, scipy=True)
         uir = matrix.tocsr()
         uir.data *= self.weight
+        if getattr(self.delegate, 'item_factors', None) is not None:
+            _logger.warn("implicit algorithm already trained, re-fit is usually a bug")
 
         _logger.info('training %s on %s matrix (%d nnz)', self.delegate, uir.shape, uir.nnz)