Version:            (using KDE 4.3.1)
OS:                Linux
Installed from:    Gentoo Packages

Steps to reproduce (in my case at least):

0) We have a weather applet containing two weather entries with tooltips containing text "heavy rain shower" (long one) and "sunny" (short one).

1) Hover the cursor to the icon with the rain picture (see screenshot).
2) Tooltip appears (and it is drawn ok)
3) Then I move the cursor to the next icon, so tooltip hops there fastly and text is changed

*) If I hover the cursor to the "sunny" icon directly, tooltip appears to be correctly drawn, so this happens only on fast switching from one tooltip to another.