Create a page titled "A (ampersand) B", substituting the ampersand character for the (ampersand). Include some recognizable contents. Save it.

Edit another page and insert an include macro to include the page A (ampersand) B. (If you wish, select the page from the search-ahead dropdown menu - it doesn't affect the bug.) After saving the macro, click the macro box in the edit pane, displaying the buttons "Edit", "Go to Included Page", and "Remove".

Click "Go to Included Page" - it should take you to the page but instead it takes you to Page Not Found.

Examining the URL on the Page Not Found browser tab reveals the title from the "Page to Include" text box was used verbatim. Correct would be to urlencode it (or otherwise load the link with a viable URL).