As far as it being a bug waiting to happen, C already has plenty of traps like that - '=' vs '==', '&' vs '&&', and so on.
In terms of grammatical clarity, I shouldn't think it would be any more confusing than any other operators that have both unary and binary versions. By my count C already has three of those: '-', '&', '*'. The latter two are even examples where the unary meaning and the binary meaning are wholly unrelated.
Besides, it's a way of using + that is already well-established in everyday idiom. And there are semantically-related uses for + that are already well-established in computer languages, too - the Kleene +, for example, should be familiar to everybody.
In terms of how it would work for the grammar I realize this would be somewhat of a departure since it's repurposing a symbol that's normally an operator to be used in a manner that would be more like a keyword according to C syntax. . . but I think that's a detail that should be much more interesting to standards committees than it is to people who primarily just use the language. And in terms of the human factor I submit that it's much more workable than taking a word from the English language and repurposing it to mean something that's more-or-less the opposite of what it means in English.
And C programmers are already spending plenty of their time cursing the world because of those things. Let's not add more of that, shall we? ;)
= and == should have different meanings.
There are two operators, equality = and assignment :=
But where it is clear that assignment was meant, = is treated as assignment. this means you can:
a = 5; -- exactly equivalent to a := 5 here
if a = 6 then
raise exception 'this should never happen';