When you are on a node page (for example node/10) and you submit a comment and get a validation error, you are taken to a different url (for example comment/reply/50).
A lot of things rely on the path, and because you are on a node page but the path is comment/reply/[nid] things break.
For example, you no longer have some classes that were based on the path, and themes relying on this class will break.