In bin/cassandra the comments say:
The lowest priority search path is the same directory as the startup script...
However the "same directory" currently has the highest priority:
# Locations (in order) to use when searching for an include file.
for include in "`dirname "$0"`/cassandra.in.sh" \
if [ -r "$include" ]; then
It looks like around the release of v 2.0.0 the order was changed but the comments stayed the same.