Hovering mouse over a job name brings up a context menu. Clicking Build Now from the context menu produces the following warning and a Proceed button. Build starts only if Proceed button is pressed. Context menu is accessible at least in job list view (e.g. default All view) and from the breadcrumb. Clicking Build Now on the left menu on a job page works normally, i.e. without warnings.
You must use POST method to trigger builds. (From scripts you may instead pass a per-project authentication token, or authenticate with your API token.) If you see this page, it may be because a plugin offered a GET link; file a bug report for that plugin.