When TeX is building a LTR paragraph, if there are some overfull \hbox s, then they stick out at the right of the offending box (please see the example on page 27 of The TeXBook and Knuth explanation).
When one is typesetting a right to left paragraph, it is expected that if there is any overfull \hbox, then it should stick out at the left of the offending box (opposite to what happens in LTR) but in TeX--XeT model, the beginning of the paragraph sticks out which is completely wrong and very annoying. LuaTeX's behavior is ok. I will file a bug report on sourceforge later.
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