Spark plugs involve many dangers: hot engines not cooled for hours; dropping a tool inside engine/belts; forgetting which wire for which plug; setting too-wide gaps causing high-speed misfires; buying wrong plugs; dropping objects in open plug holes.
Biggest problem is “hot engine”: driving from the auto-parts store and not waiting hours, with open hood, for the engine-block to cool. Most people are too busy to wait, but spark plugs are about the last thing to cool. If desperate, use gloves to reach easiest plugs, while back plugs cool more. Remember engine heat also fries patience and boils tempers.
Dropping tools: After decades of car design, you might expect a built-in work rack to hold tools near an engine: don’t. Many engines seem designed for all to slide, break and kill. Use your own tool-tray if possible, but ALWAYS beware dropping tools in the engine with moving belts.
Hard-to-reach plugs: Newer cars seem to have plugs that are difficult to reach, so pre-look for all plugs to see where to reach. Consider replacing hidden plugs second (while still having patience), before replacing easier plugs.
Count your tools when done: It is too easy to overlook a wrench, socket, gap-gauge, or old plug perched inside the engine. Memory fades after replacing 6 or 8 plugs. Count 6: wrench, plug-socket, pliers, gap-gauge + perhaps 2 old plugs.
Fuel-fouled plugs: If the engine has run without some plug firing (perhaps forgot to re-attach a plug cable), fuel collects there, flooding the plug. An engine might run almost a full minute to burn the collected fuel under that plug, and run smoothly again. Just remember: a “lot of fuel burns a lot of air” (more than just a few cycles of air).
Wrong plugs: Triple-check spark plug model numbers. Contrary to obvious naming, plugs are often numbered with dull numbers, 45 & 46, or the same forgettable digits merely swapped: such as “5245″ or “2425″ (there’s no memorable “1973″ or “1776″; at least biscuits had “1869″). Write it down & triple-check before buying: or you might think “5224″ was it, and then drive a hot engine to get other plugs.
Falling inside: Make sure when replacing spark plugs that nothing falls into the hole the spark plug is threaded into. Use compressed air to blow away dirt and debris right before you remove an old plug. Fix: If dirt fell in, consider starting the car without that plug, allowing the piston to force air/dirt out in loud bursts.