I get this assert when doing a parallel_for: tbb::task_group_context context(tbb::task_group_context::isolated); context.set_priority(tbb::priority_high); tbb::parallel_for(range, functor, tbb::auto_partitioner(), context); Values of local variables: my_global_top_priority = 1 a.my_top_priority = 2 We have multiple normal threads that might do parallel tasks with either normal or high priority. Usually, high priority tasks occurs from threads with higher priority. and are meant for fast user input responses. Call stack looks something like this: tbb_debug.dll!tbb::assertion_failure tbb_debug.dll!tbb::internal::market::lower_arena_priority(tbb::internal::arena & a={...}, __int64 new_priority=1, __int64 old_priority=2) Line 459 C++ tbb_debug.dll!tbb::internal::arena::is_out_of_work() tbb_debug.dll!tbb::internal::custom_scheduler::receive_or_steal_task(__int64 & completion_ref_count=2, bool return_if_no_work=false) Line 302 + 0x11 bytes C++ tbb_debug.dll!tbb::internal::custom_scheduler::local_wait_for_all(tbb::task & parent={...}, tbb::task * child=0x0000000002156740) Line 548 + 0x69 bytes C++ tbb_debug.dll!tbb::internal::generic_scheduler::local_spawn_root_and_wait(tbb::task & first={...}, tbb::task * & next=0x0000000002159bc0) Line 629 + 0x35 bytes C++ tbb_debug.dll!tbb::internal::generic_scheduler::spawn_root_and_wait(tbb::task & first={...}, tbb::task * & next=0x0000000002159bc0) Line 536 C++ OurLib.dll!tbb::task::spawn_root_and_wait(tbb::task & root={...}) Line 693 C++ OurLib.dll!tbb::interface6::internal::start_for<...>::run OurLib.dll!tbb::parallel_for