The perception of a bistable stimulus as one or the other interpretation can be biased by prior presentations of that stimulus. Such learning effects have been found to be long lasting even after small amounts of training. The effectiveness of training may be influenced by preexposure to the ambiguous stimulus. Here we investigate the role of preexposure for learning a position-dependent perceptual bias. We used rotating Necker Cubes as the bistable stimuli, which were presented at two locations: above or below fixation. On training trials, additional depth cues disambiguated the rotation direction contingent on the location. On test trials, the rotating cube was presented without disambiguation cues. Without preexposure to the ambiguous stimulus, subjects learned to perceive the cube to be rotating in the trained direction for both locations. However, subjects that were preexposed to the ambiguous stimulus did not learn the trained percept-location contingency, even though the preexposure was very short compared to the subsequent training. Preexposure to the disambiguated stimulus did not interfere with learning. This indicates a fundamental difference between ambiguous test and disambiguated training trials for learning a perceptual bias. In short, small variations in paradigm can have huge effects for the learning of perceptual biases for ambiguous stimuli.