A misidentified species of extinct Mauritian comb duck was initially described from unrecognised remains of the Mauritius sheldgoose (Alopochen mauritiana); this was realised as early as 1897,[3] but the printed case of mistaken identity can still, occasionally, be found in modern-day sources.

[4] Analysis of mtDNA sequences of the cytochrome b and NADH dehydrogenase subunit 2 genes, however, suggests that it is a quite basal member of the Anatidae, vindicating the earliest placement.

Immature knob-billed ducks look like a large greyish female of the cotton pygmy goose (Nettapus coromandelicus) and may be difficult to tell apart if no other birds are around to compare size and hue.

However, knob-billed ducks in immature plumage are rarely seen without adults nearby and thus they are usually easily identified too.

[4] This duck feeds on vegetation by grazing or dabbling[4] and to a lesser extent on small fish, invertebrates, and seeds.

[9] The knob-billed duck is declining in numbers locally, but due to its wide range it is not considered globally threatened by the IUCN.