CCtx::try clone (experimental feature, deprecated) copies tables sized from the context's applied parameters. Starting a multithreaded frame (zstdmt feature) replaces those parameters with the workers' without resizing the context's own tables, so after an abandoned single-threaded frame, the copy wrote up to 512 KiB past a heap allocation, or read past one. This became reachable with the bundled zstd 1.5.6.

The flaw was corrected by refusing try clone (and the unsafe compress block) once NbWorkers was set on the context.