Shell thickness and molecular marker data indicate that illegitimacy and contamination are more widespread in oil palm breeding programmes than is usually acknowledged. Before the discovery of the mode of inheritance of shell thickness, no markers were available to detect illegitimacy. Once shell thickness could be used as a marker, it became clear that control of pollination needed improvement. However, in present day programmes anomalous segregation ratios and contamination with the wrong fruit forms still occur from time to time, and work with molecular markers shows that illegitimacy may occur even when fruit form segregation is correct. Where there is illegitimacy, family selection will be unreliable, and inadvertent inbreeding may take place. Now that molecular markers are widely available, it should be standard practice in oil palm breeding to test all crosses for legitimacy. The requirements for this are discussed.