tartar » tarter
Spotted in the wild:
Pointed out to me by Ann Burlingham on 26 September 2008; she was astonished by the number of occurrences of “tarter sauce” she’d googled up (after noticing an instance). I got 76,800 raw webhits (vs. 444,000 for “tartar sauce”).
Many of these are undoubtedly just phonetic spelling, using the much more frequent spelling -er. But I suspect that some of them are facilitated by the idea that the word contains the adjective “tart”.