This only occurs on the splitbrain/doku site. It does not occur on my 2005-09-22 or 2005-07-13 sites.
I'm running IE 6.0.2800.1106. The problem is not due to a flaky browser instance or bad connection, because it's happened two days in a row, with a hard boot of my machine in between.
I currently have three browser windows pointing at pages on the Dokuwiki site; one is a search results screen, the second is wiki:discussion:manageprofile, the third is the edit page for wiki:discussion:manageprofile. The search was for "profile button" (with the quotes), and that's how I got to the discussion page.
In the edit window, if I click the Insert Signature button, or try to insert a Smiley, the cursor blinks, the Save button blinks as if it had been clicked, but nothing appears in the edit window. But that doesn't mean nothing is happening! Quite the contrary. My signature is being pasted into the Search Results page, above the title, where it says "[[profile_button]]"! Very cool, but not very useful.
I should probably mention that the Search Results window is the original. The others were opened by right-clicking on it and using Open Link In New Window. (I do that a lot because your site is very slow to respond, even to the Back button.) I'm not sure if it's important, but it certainly could be. I ran a quick test with a browser window that WASN'T opened that way, and the signature worked.
I'm attaching a screenshot of the result. I assume you can open a bmp file.