The truly paranoid worry that the processor will not execute instructions as they think it will. This ranges from timing irregularities from branch prediction a

The truly paranoid worry that the processor will not execute instructions as they think it will. This ranges from timing irregularities from branch prediction and instruction re-order to bugs in CPU microcode or deliberate backdoors in the the design, or even the fab altering the mask to change the design. https://eprint.iacr.org/2007/039.pdf https://eprint.iacr.org/2007/039.pdf http://www.cl.cam.ac.uk/~sps32/microsemi_re.pdf http://www.cl.cam.ac.uk/~sps32/microsemi_re.pdf