Yep. Nine months to change a single buggy line of cache line invalidation in the translation look-aside table handling IMUL opcode in QEMU 4.0.

    imul eax, [ecx+0x41], 0x10

Entails hundreds of hours of single-stepping through that opcode in Linux kernel using an indirect operand pointing toward its own opcode (self-modifying code).

Even the extraordinaire Fabrice Bellard (author of QEMU) admitted that it is broke and did a total rewrite, which fixed tons of other issues.

https://github.com/unicorn-engine/unicorn/issues/364