Newer Things that make you say Ouch!.

Tubes can hurt you dead. Don't play with them unless you know what you are doing and respect the danger involved.
Don't work on high voltages alone, someone has to be around to at least laugh at you when you get shocked and have to go change your underwear (or worse, they need to douse you with a fire extinguisher.)

OSHA says dangerous voltages start at 50V peak. 

Voltages below 50V can hurt you too. At 24V DC I've seen co-workers be electrically shocked and I have seen wire throw shrapnel feet, not inches, from failing under high currents.

Opening an inductive circuit with current flowing will generate high voltages. When a part or wire fails, it can generate high voltages.

Touching a wire that is smoking because of too much current will burn you. It can cut through your "meat" like a hot knife through warm butter. I watched a buddy try to unplug a smoking wire and ended up having to drive him to the ER.

I'm not your guardian , you are responsible for your own safety if you try anything I'm discussing or linking to in these web pages.

These design ideas come with negative warranties,
        if you try them, your warranty is void!

Manufactures may change their designs over time and some manufactures do not change their model number when the make the change. Before you make a change, make sure your version of your equipment matches the version the change was written for.

This means before you attempt a modification, make darn sure the stock version is working correctly first. If the stock version has a problem, there is a chance the manufacture will fix it for you. If you do anything not in the manual and it does not work, it is your challenge to fix it, not mine nor the manufacture's.

For example: If you hear that putting elephant snot on your circuit will make it perform better, it is your problem to make the circuit work with the elephant snot on it. Nobody, not me, nor the manufacture, is required to touch a circuit with elephant snot on it. Once elephant snot touches your circuit, your warranty is void. This same concept applies for mods that do not involve elephant snot.

By the way:

1. If you are close enough to an elephant to see its nose without binoculars, you are too close for safety!

2. The odds are 10:1 that real elephant snot is a controlled substance.