The file /etc/gtk/gtkrc, bundled in the SRPM for gtk+, causes GTK tooltips to be incorrectly displayed in some cases. It would be best if a suitable foreground color (such as black) would be assigned along with the background bg[NORMAL] = "#ffffc0" present in this file. The problem occurs if fg[NORMAL] is redefined for a broader subset of widgets to some very light color (such as white) in the per-user gtkrc file. The end result is then white on light yellow, which is unreadable. This is also a problem in updated gtk+-1.2.6-1 from the 6.1 errata.
I've now set bg[NORMAL] to black in the current package. I'm not completely happy with this fix, since it a) tooltips will be black/yellow in all cases b) tooltips with pixmap backgrounds may not display properly But it is about as good as can be done easily. (The right approach is possibly to have a "default" theme, which read only if another theme isn't read.)