A customer was complaining about their rough running engine. After a quick test run, I knew the ignition system was having problems. More specifically, I thought the spark plugs were fouled. After pulling a few of the plugs I found this one:
A tiny amount of lead had solidified on the iridium tip and closed the gap creating a dead short, thus no spark would be made. Such a small thing, and yet it created so much frustration for the customer. Luckily it was a quick fix.