The suffix "-penia" as commonly seen in words such as thrombocytopenia, leucocytopenia, lymphopenia, etc. is commonly incorrectly spelled "-paenia" or even occasionally "-poenia" (incorrect derived forms using the suffix "-paenic" are also seen). This derives from the mistaken belief that the suffix penia is an American spelling variant of an original British English spelling, paenia. In actuality, the term penia derives from the Greek word πενία (penia) meaning 'poverty' 1.
This phenomenon, which is well known to linguists, is called hypercorrection. Hypercorrection is the misapplication of a linguistic rule to the wrong situation, in this case the rule that many American spellings with an 'e' are spelled with an 'ae' in British English, e.g. pediatrics instead of pediatrics.
Indeed there is a recent neologism for the phenomenon by which some British English speakers tend to detest American English to the extent that they would make an ignorant fallacious change of this kind, amerilexicophobia 2.
Other words for which some individuals and journals (erroneously) change the spelling to a non-existent 'ae' form are
stapedial: stapaedial
venepuncture (or venipuncture): venaepuncture
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