The HP 41 had something called "synthetic programming", a flaw that someone once found in the calculator that allowed access to a lot of its internals, including brand new opcodes and capabilities (even graphics, which the HP 41 didn't officially support).

The downside was that doing the wrong thing could cause the calculator to shut down sometimes for days in a row. I once did that to my father's calculator and these were the longest days in my life, dreading that I might have broken his $300+ purchase for silly programming antics.