If the security policy is configured appropriately, a user might be able to add a comment on a Wiki Page:
Page comments/add-comment-1.png

Page comments are an special type of page edition, that is, the comment is stored inside the page, at the end, with a custom markup, and with no page content being removed. But having edit permissions on the page, the same effect can be achieved though a "normal" page edit.

The comment editor is pretty similar to the main editor, although there are some small differences:
Page comments/add-comment-2.png

The most important ones are the comment signature + the URL field. These are used to add a signature following the comment, but keep in mind that they are free fields, not tied to the user making the comment. You may use whatever name you see fit. As the comment, at the end, is no more than a page edit, the 'true' author of the comment will be visible on the Page Info.

Given the above, Wiki Trolls may abuse the comment permission and make a comment using someone else's name. Comment author will be always visible through the Page Info; remember: a comment signature does not necessarily denote who made the comment.

As with other permissions, Administrators should give some care to think who they're trusting with this permission.