Goto: http://tinymce.moxiecode.com/example.php?example=true Select some text and click the strikethrough button. Nothing happens. Anders implemented execCommand(strikethrough, ...) (6691), but TinyMCE doesn't seem to use it.
While masquerading as Mozilla 1.1, I tested this at the specified URL and strikethrough worked as expected (for the most part*). This is using revision 15077. *The broken part is not just strikethrough. I can't reproduce it consistently, but after applying effects using toolbar buttons, the selection sometimes jumps around wildly and then can't be unselected. The selection appears to be active, but it's like it was painted but forgotten about. In order to get rid of it, you need to reselect all the offending area and then click somewhere to deactivate the selection.
Fixed in dev versions of TinyMCE, see for example http://tinymce.moxiecode.com/test/examples/example_full.htm