1. I was told that Unix couldn't dual boot with MS-DOS... so I patched the boot sector to load an alternative version of itself into RAM before system start if an unused bit was set (thus enabling DOS to boot)... so I could reboot back and forth... sometime around 1985.
2. Built a box with a Z80, 2764 EPROM, A/D converter, speech chip and a hacked together telephone interface... had 4 inputs and read the voltages of each to the caller on the phone, twice... then hung up.
3. Wrote a Forth for OS/2 in assembler... because I was told you couldn't write assembler programs in OS/2.
4. Built a system out of solar cells behind a filter, to detect infrared laser, and help align laser CATV links, with a companion box to generate a tone to feed into the transmitting laser.
5. Used a bi-color LED as light and sensor to detect a beam break to a reflector. (Green light can be detected by the red LED, but not vice-versa)