I think that strictly speaking this is not an error, because the expression returned by SELF:odbKontroll:FIELDGET(#Kontroller) can be a number (since FIELDGET returns a usual), and the +/- unary operator is valid for numbers. Using the plus sign might sound strange, but it's the same as using the minus sign, which is valid in the case of a number:
This is adding the negative value of the field, of course it would make more sense to use -= in the first place, but still, this is valid syntax.
Of course in your case you are using a string variable, in which it is not valid to add/subtract a number, but I don't think the compiler can be made that smart to recognize this (since it does not know the return value type of FIELDGET). Unless Robert has a different idea..