The following steps crash Safari with the latest debug build. The default build of Safari seems to have no problem with this.
Open Safari from terminal
Click the Back button
Safari crashes with:
ASSERTION FAILED: ![_private previousItem] (/Users/vadim/WebKit/WebKit/WebView/WebFrame.m:1243 -[WebFrame(WebPrivate) _recursiveGoToItem:fromItem:withLoadType:])
*** This bug has been marked as a duplicate of 6454 ***