1. An integrated circuit chip comprising: system circuitry configured to perform a function, the system circuitry comprising a plurality of sub-circuits configured to perform the function by performing concurrent actions; debugging circuitry configured to detect one or more errors in the system circuitry's performance of the function, the debugging circuitry comprising variability circuitry; wherein the debugging circuitry is configured to functionally test the system circuitry by: at the variability circuitry, altering relative timing of the concurrent actions so as to increase the likelihood of one or more errors in the system circuitry's performance of the function; and recording one or more errors.