Actually, this is exactly the assumption that led to the Meltdown bug. The Intel CPU speculative (non-retired) instruction pipeline runs instructions which access memory and load it into cache and execute the instructions, delaying the check that the memory is accessible within the current context until later (just before instruction retirement).

It's certainly possible that a CPU could delay the same sort of exception processing for "NX" flags until later in the pipeline, but still before retirement, allowing a similar sort of problem as Meltdown.