It's surprising that most languages will happily let you do this. Lots of C programs have 'FOO' and 'foo'; not so many will have 'Foo' as well but it's probably more common than you think.
IMO this is only a problem if they both are the same type, e.g. both are a function, both are a type, both are a variable, both are a special keyword, things like that, because then it's hard to mix them up.
If however you can substitute one for the other in code and still have it compile, that would be terrible.