This script is a copy of User:Ucucha/HarvErrors, which checks for possible mistakes (broken anchors) in Harvard citation template calls. It is modified to show each message in a tooltip of a warning icon (
or 
, depending on the type of error; the exclamation mark is for links that don't point to any citation, and the question mark is for citations with no links pointing to them).
If any harv errors are found, a link ("Go to first harv error") is added to the "Toolbox" section of the sidebar, pointing to an anchor where the first error is located.
The main function has been given a name (checkHarv) so it can be called from other scripts, such as the ajaxPreviewExec feature of User:Js/ajaxPreview.
It is not recommended for users who browse from mobile platforms or have special accessibility requirements. However, the warning message is included in both the title and alt attributes of each icon, so some screen readers may be able to read it. (I have not yet tested any screen readers.)