What the heck...? It's most definitely a problem in practice! That problem becomes pretty obvious once you get a vocabulary of around 10k ~ 15k words and you read both fiction and non-fiction books. What you find is that every time you encounter an unfamiliar word you only really have a best guess at how to read it and you actually can't be 100% sure. This has repeatedly been my experience. I spent some time thinking about if I could shortcut this process by instead simply learning all the "exceptions to the rule" but what I discovered is that the volume of them is too large for that to be a tractable approach.