The following scenario can be seen in locations, where you can add credentials through the credentials plugin.

The "+ Add" button is a jenkins-button, as it should be, but its wrapped into a span adding yui-button on top of it, which displays two buttons, like we can see on the screenshot. The grey background is the hover effect of the jenkins-button and the blue frame is the hover effect of the yui-button.

I'm not exactly sure where the credentials plugin adds the YUI buttons on top, but that's no longer needed, given the credentials plugin already utilizes the modernized buttons.