The Ultimate Reduced Instruction Set Computer that only executes the "subtract and branch if negative" instruction. Code (or should I say data, since there's little need for an instruction word) written for this... vile abomination is necessarily self-modifying and it is thus not known to the author if URISC-style processors are turing complete (an example of a programming language that never was turing complete is Malbolge, where a jump backwards would modify an important byte rendering the jump idiom impossible to execute again).
Sure as hell wouldn't need a code cache or a complex instruction pipeline on an URISC CPU, if someone actually were crazy enough to implement it in silicon.