If it's Fuji Acros you'll be fine; some others, not so much. In my (limited) experience, the lighting conditions that lead to super long exposures often have a wide range of scene brightness, and the exposure is essentially selecting what part of the range to display. As such, I almost always "get something," it just may not be quite the way I visualized it.
Originally Posted by mporter012
Having just come off Worldwide Pinhole Photography Day and having accidentally exposed a sheet of ISO 100 film as though it were paper at ISO 6, I've had recent experience! (In that case, reciprocity failure worked in my favor.)