Previous: x-symbol interoperation, Up: Known problems
This is probably the fault of your favorite package. flyspell.el and mouse-drag.el are known to be affected in versions before Emacs 21.3. Upgrade to the most recent version.
isearch.el also shows this effect while searches are in progress, but the code is such a complicated mess that no patch is in sight. Better just end the search with <RET> before toggling and resume with C-s C-s or similar afterwards. Since previews over the current match will auto-open, anyway, this should not be much of a problem in practice.