There has been confusion surrounding this taxon and N. stenophylla ever since it was first described. N. fallax matches N. stenophylla in most respects, except for the shape of the lid; the type specimen of N. fallax has an orbiculate lid, whereas that of N. stenophylla is narrow. However, the description of N. stenophylla was based on a plant raised from seed in a greenhouse in England, and the shape of the lid could be an aberrant characteristic resulting from artificial growing conditions.
In his seminal monograph "The Nepenthaceae of the Netherlands Indies", B. H. Danser treated N. fallax as a junior synonym of N. stenophylla. This interpretation was supported by a number of subsequent authors.
The following natural hybrids involving N. fallax have been recorded.