Thread: Can't convert from 'unsigned short ** ' to 'const unsigned short ** '? Why not?

Code:

cannot convert parameter 2 from 'unsigned short ** ' to 'const unsigned short ** '
Conversion loses qualifiers

I've seen this error a few times and it kind of baffles me.
I know you need a const_cast to remove constness, but if I'm reading this correctly, this error says it can't add constness.
I've passed non-const parameters to functions that take const parameters more times than I can count without problems. Why would it complain about this only sometimes? I thought constness can always be implicitely added, but you have to explicitely remove it?