When tracking third party source, it is useful to have it on separate branch and merge onto trunk as needed.
This doesn't work in Fossil:
<pre>
rm -rf /tmp/fossil-merge-bug
mkdir /tmp/fossil-merge-bug
cd /tmp/fossil-merge-bug/
fossil new bug.fossil
mkdir bug
cd bug
echo "vendor version" > test
fossil open ../bug.fossil 
fossil add test
fossil ci --branch vendor -m "Import vendor source."
fossil close
rm -rf *
fossil open ../bug.fossil trunk
echo === Attemt the merge... ===
fossil merge vendor
</pre>

The script output here is:
<pre>
project-id: 3f2182fb3f096d5197fa8788ff73a14c62d9b7b9
server-id:  d908e77a1322004ebae2cceeb10ea32689ee611d
admin-user: asau (initial password is "485164")
ADDED  test
New_Version: f7f55946e4adbab2523aa38f08e15e519b16ce01
project-name: <unnamed>
repository:   /tmp/fossil-merge-bug/bug.fossil
local-root:   /tmp/fossil-merge-bug/bug/
project-code: 3f2182fb3f096d5197fa8788ff73a14c62d9b7b9
server-code:  d908e77a1322004ebae2cceeb10ea32689ee611d
checkout:     10a4088889fec65a08f69899969e3e8e51a9de8d 2010-10-14 13:54:22 UTC
child:        f7f55946e4adbab2523aa38f08e15e519b16ce01 2010-10-14 13:54:23 UTC
tags:         trunk
comment:      initial empty check-in (user: asau)
=== Attemt the merge... ===
fossil: lack both primary and secondary files
</pre>