On master commit ed1b2b2
I get a crash with an assertion failure at line 1112 in paragraph.py, the assertion failes calling container.advance2(descender)
The problem is that the container has just enough height left, but there's a floating point rounding error:

(Pdb) p float(container.remaining_height) + descender
-5.329070518200751e-15

I'd prefer not to share the source files, although we could talk about doing so privately if it's really necessary.
However, whenever there's a chance of a floating point comparison needing exact equality, you need to account for rounding error like the above.
I think the appropriate fix would either be in advance2 or in the equality operators of the dimension objects depending on how global of a fix you want.