This one was fairly recent. I got a call from a local customer first thing in the morning that they had a line that had been down since 3am. Talking through it over the phone, they had an SMC EX260 that was failing to come online. The exact part number was a custom, so I crossed it to a standard part that a local distributor had on the shelf. After picking one up and heading to the facility, I saw the aftermath of the 3rd shift maintenance group. It seems they originally thought that just a valve was bad, so they began swapping valves between manifolds to isolate it. They thought that since they saw either both lights on a valve, or none depending what they hooked up, that they had found the problem. They didn’t realize that in some cases they had swapped double solenoid J valves with either exhaust or block center valves. There were air hoses for each section, many different sizes, just lying on the floor next to the corresponding valve bank. After a few quick checks that the old SMC EX260 was indeed bad, we quickly replaced it. It was online within 30 minutes of my arrival. We spent the next 4 hours going through the logic and the line to put the correct valve types in the correct order in each bank, and the correct tubes hooked up to each valve. This time the plant maintenance labeled the hoses as we put them back in place.
Bottom line; something as simple as labeling or taking cell phone photos as you take things apart can save hours of downtime and frustration.