Proper orientation of cocoons in linearly structured nests can be crucial for the survival of hatching bees. Nevertheless, misoriented cocoons appear in nature in notable proportions. A detailed analysis of sex, space available for cocoon spinning, cocoon size, and nest diameter in the red mason bee (Osmia bicornis) showed that smaller males are more prone to misorientation than larger females. Generally, smaller individuals of both sexes are more often misoriented because smaller larvae disregard the direction of the entrance during cocoon construction. Moreover, cocoons in stems of smaller diameter are less often misoriented than bees in wider nests. Our results suggest that cocoon size and available space are the most important traits affecting cocoon orientation.