Comment by adrian_b

Split locks are never something anyone wants to do, unless they are morons.

Split locks are always bugs, but like many other bugs that are benign on Intel/AMD CPU-based computers (due to efforts done by the hardware designers to mitigate such bugs, instead of crashing applications), they are not uncommon in commercial software written by careless programmers.

The Intel/AMD CPUs contain a lot of additional hardware whose only purpose is to enable them to run programs written by less competent programmers, who also use less sophisticated programming tools, so they are not able to deal with more subtle things, like data alignment or memory barriers.

This additional hardware greatly reduces the performance impact of non-optimized programs, but it cannot eliminate it entirely.