An erroneous plural, which I offer for comparison is is:
syntagma
It has a rough comparability in frequency to the less exotic, more naturally English sounding 'syntaxes''syntaxes'.
This is erroneous as a plural of 'syntax' because it is a linguistic term having to do with rule based things but it is not a word to describe a multiplicity of a collection of rules. 'Syntagma' is a sequence of words that form a syntactic unit (for example, a noun phrase), a -single- syntactic constituent (no rule or rules, the plurality is in the (expected) number of words).
Note: I have no explanation whatsoever of the shifted character of the graphs.