The problem here is that tracing for STACKADJ prints out the top of the stack *after* adjustment. This is OK for growing the stack, but not for shrinking it (e.g. calling STACKADJ(-3) pops off three things at once, which can leave TOP undefined when it's printed out).

On the EuroPython sprints, I recommended splitting STACKADJ into STACKADJ_GROW and STACKADJ_SHRINK, since the printing behavior should be different.
This is performance-critical code; it needs to be reviewed carefully.