PDF files created by Inkscape are causing Xelatex to crash.
This happens when a figure includes gradients.
The compilation process exits with the message:
** ERROR ** pdf_link_obj(): passed invalid object

https://bugs.launchpad.net/ubuntu/+source/libcairo/+bug/1051939

As discussed in this thread, the error seems to appear for PDF file
with broken references:
http://tug.org/pipermail/xetex/2010-December/019653.html

Such files seem to be fixed by running "pdftk".

Steps to reproduce:
Create a figure with a gradient in Inkscape and save it as PDF.
Include this PDF in a latex document and try to compile it with xelatex.

(most of this bug report is borrowed from launchpad bug)