Supercede v Supersede (Merriam Webster v OED)
According to the Shorter OED (5th Edition):
“supersede: Also (earlier) -cede L15. [Old French superceder, later -seder, from Latin supersedere (in medieval Latin freq. -cedere) set above, be superior to, refrain from, omit formed as SUPER- +sedere sit.]”
Merriam Webster (online) asserts:
“Supercede has occurred as a spelling variant of supersede since the 17th century, and it is common in current published writing. It continues, however, to be widely regarded as an error.”
So, both prefer -seded, and this is closer to the root. MW admits common usage then sort-of asserts -cede to be an error. OED just says -cede is a variant (not an error).
Microsoft Word, by the way, marks it as an error.
[Update: Merriam corrected]