I think Lua interpreter itself is fairly stable. But the problem is that it lacks the proper mechanism for language upgrade or deprecation---for example we have

I think Lua interpreter itself is fairly stable. But the problem is that it lacks the proper mechanism for language upgrade or deprecation---for example we have found a (undocumented) bug affecting Lua 5.1.x which has been silently fixed in 5.2, but we were stuck at 5.1 so we had to fork the interpreter. That kind of things. The development process of Lua is definitely flawed in my humble opinion.