Using David Ponce's tabbar.el, I invoked customize-face, unchecked the
Background attribute of tabbar-default-face and saved the
customization. This resulted in a stream of messages alternating
between "Invalid face background" and "Invalid face foreground". The
messages were not displayed in the echo area but only in the
*Messages* buffer. Rapidly scrolling the *Messages* buffer while
these messages were being recorded caused Emacs to crash. Rechecking
the Background attribute of tabbar-default-face and saving the setting
eliminated the messages and scrolling the *Messages* buffer no longer
caused Emacs to crash. This (both the crash and the stream of
messages) has only happened with this particular customization;
unchecking the Foreground attribute, or unchecking or otherwise
customizing the Fore- or Background attributes of the other tabbar
faces, has been innocuous.