I’ve heard this twice last week. Friends and customers had purchased a pin based bulb and tried to replace their existing burned out bulb.
When they inserted the lamp into the fixture it wouldn’t go in. The fixture seemed the right size and shape but the pins were the wrong size and wouldn’t go in to the socket.
What’s going on they asked me? One had a 2D lamp and they other had a 4 pin quad lamp. Although different styles of bulbs, they were both pin based. The answer for both of them had to do with the lamp being the wrong wattage. Pin Based lamps have one disadvantage: they are wattage dependent. So you have to put a lamp of the same size wattage into the fixture. If you want a different wattage e.g. going from 55W to 38W, you will have to change out the fixture. So in the case of the friend with the 2D Lamp, she needed to put in a 55W 2D bulb not a 38W.
This is an issue you don’t face with GU24 fixtures. It’s easy to swap out a bulb of a different wattage. See last week’s post on the advantages of GU24 base bulbs.