If you fail to specify a --llvm-symbolizer value when building with --sanitize=thread, you'll get an UnboundLocalError:
scons VARIANT_DIR=thread CC=`which clang-3.6` CXX=`which clang++-3.6` --sanitize=thread -j7 --allocator=system --opt=off core ... UnboundLocalError: local variable 'tsan_options' referenced before assignment: File "/home/s/code/mongo/mongo/SConstruct", line 2202: env = doConfigure( env ) File "/home/s/code/mongo/mongo/SConstruct", line 1828: tsan_options += "suppressions=\"%s\" " % myenv.File("#etc/tsan.suppressions").abspath