It depends. Off by one errors are common, but the context is important. Some languages for example prefer integrators to indexing, and if ‘a’ was an index that might have been avoided.

Languages with algebraic data types also allow you to express state in ways that can make it easier to identify state changes, possibly making the missing increment more obvious.

Some languages will warn when variables are being used or modified in scopes where they are referenced but not used.

But, even with all this, yes bugs will still happen.