The Lock Transparency icon (which looks like a checkerboard) at the top of the Layers palette gets in my way most often (because I forget it’s turned on). Lock Transparency prevents you from changing the transparency of areas. Each layer has its own Lock Transparency setting. Therefore, if you turn on the Lock Transparency icon for one layer and then switch to another layer, the Layers palette will display the setting for the second layer, which might be different from the first one.
Try using the Eraser tool when Lock Transparency is turned on—it will mess with your mind! Because the Eraser tool usually makes areas transparent (by completely deleting them), it will start painting instead when Lock Transparency is turned on. It will fill any areas you drag over with the current background color. However, if you paint across an area that’s transparent, it doesn’t change the image at all (because the transparent areas are being preserved). You can see how it can get in your way if you forget you turned it on.
Try this: Open a photo, and delete areas around it using the Eraser tool. To accomplish this, you’ll have to change the name of the background first (you can’t poke a hole in the background, but you can on a layer); then make sure Lock Transparency is turned off. Otherwise, you can’t make areas transparent. Now use the Eraser tool to remove the areas that surround the subject of the photo, and then choose Filter > Blur > Gaussian Blur and use a really high setting. You’ll notice that the edge of the image fades out and blends with the transparent areas surrounding it. Now, choose Edit > Undo and try doing the same thing with the Lock Transparency option turned on. Notice that the edge cannot fade out because Photoshop will not change the transparency with this option turned on.