When a multithreaded frame (zstdmt feature, CParameter::NbWorkers) is left unfinished, its jobs may still be running on the worker threads: the C library only waits for them when the next frame starts, or when the context is freed. A session reset makes the context accept changes again in the meantime, so:

The flaw was corrected by refusing these calls while a multithreaded frame is unfinished. The new CCtx::wait for workers() waits for the workers first.