I had a customer ask me to replace the lamps in his parking lot, 11 all together. So I called around to a few different shops that do this type of work to get an idea of market conditions. Not being sure if the lamps were 250watt or 400watt I order both.
So I ordered the lamps, rented a lift and got to work. As I was taking the bulb out of the first fixture it (the bulb) fell apart. I compared the general size of the old lamp to the new 250watt lamp, looked the same, so I put it and the other 10 lamps in, turned on the contactor and all was good.
Now as I was packing the old lamps into a box I happened to look at one of the old bulbs and it read 400watt !! Uh oh, that's right, 400watt. That's when I learned there is such a thing as a reduced envelope lamp.
Turn off the contactor, call the local supply house that I don't usually deal with, get the right lamps at a higher price, and proceed to replace all of the lamps AGAIN.
Lesson learned...