One experimental finding that led to the development of feature integration theory (Treisman, 1988) was the discovery of illusory conjunctions (Treisman & Schmidt, 1982).  Illusory conjunctions occur when features are mistakenly combined.  For instance, subjects might be presented a red triangle and a green circle in a visual display, but experience illusory conjunctions: a green triangle and a red circle.  Illusory conjunctions generally occur when attentional resources are limited in some way.  This is consistent with feature integration theory, which presumes that focused attention is require to combine different features together at the same visual location.  When such attention is not available, features can be conjoined incorrectly because their locations are not fixed.

References:

1. Treisman, A. M. (1988). Features and objects: The fourteenth Bartlett memorial lecture. Quarterly Journal of Experimental Psychology, 40A, 201-237.
2. Treisman, A.M., & Schmidt, H. (1982). Illusory conjunctions in the perception of objects. Cognitive Psychology, 14(1), 107-141.

(Added March 2011)