Commodore 64 Basic, 4 bytes
===========================

    1S|0

PETSCII substitution: `|` = `SHIFT+Y`

The [zero page][1] of a Commodore 64 is an area of 256 bytes of memory that can be accessed faster than the rest of RAM.  Consequently, programs (such as the BASIC interpreter) use it for frequently-accessed data, and the CPU itself stores some of its internal state here.  The contents are subject to change without notice.

The BASIC program above, ungolfed, is `1 SYS 0`, ie. transfer execution to memory location 0.  This starts executing the zero page as code.  Normally, when the BASIC interpreter starts running a program, the first 16 bytes are

    2F 37 00 AA  B1 91 B3 22
    22 00 00 4C  00 00 00 00

so `SYS 0` would execute the following

    00: ROL-AND $37,A  - Undocumented opcode: rotate the value at memory location 0x37 left, and store the result in the accumulator
    02: BRK            - Call the interrupt vector

The overall result is to output the BASIC `READY.` prompt and return control to the user.  However, memory location 0x00 is the CPU's I/O direction register, and memory location 0x01 is CPU's I/O address register.  If you've done something that changes these before running the program, the results can be unpredictable, ranging from outputting garbage to locking up the computer (the 0x22 usually contained in memory location 0x07, if executed as an instruction, is an undocumented `HALT` opcode).

Alternatively, a more reliably unpredictable program is the four-byte

    1?TI

Prints the elapsed time, in jiffies (1/60 of a second), since system power-on.

  [1]: https://en.wikipedia.org/wiki/Zero_page