You read it right. Its not Locker Service that may break your code, but Summer 17. If you have legacy Visualforce page, which needs to navigate to Lightning experience and URLFOR is used directly, then chances of getting below error are very high.
In above screenshot, I got MALFORMED_ID : malformed Id error in one of my Visualforce, where I needed to navigate to new Account record. Continue reading “Behold Yourself – Summer 17 is here to break legacy Visualforce Navigation code in Lightning Experience”