If you have a mixture of plain and bold spans of text in a paragraph, the bold spans don't behave properly with respect to line breaking – the system will happily break a line in the middle of a bold word (though apparently this happens only for the first word in a bold span), and in particularly perverse cases will place the first character of a bold word at the start of a line and then break immediately after it.
Here's a test case I made:
<Span>This paragraph has some words in </Span>
<Span>, and if you </Span>
">reshape the window</Span>
<Span> to certain widths, they will break poorly.</Span>