I don't think that person means you literally can't know, just that it increases the difficulty of reasoning through the code.

I was debugging some code earlier today. Someone had put a global variable that is either altered or used in 4 or 5 different functions across our codebase. I had to literally draw out the paths a user could go down to figure out what the value of this global variable would be at the time I was trying to call one of those functions. It was not awesome.

I figured it out, so you're right. I do know which functions touch the variable and NOW I know when. But I still can't guarantee the value of the variable.

Needless to say, tomorrow will see a little refactoring.