Jane Howitt wrote:
Could it be something to do with the document setup?
BTW, is there another way of resizing something in PS apart from Ctrl+T?
It is most likely something she is doing. You would need a screen capture that includes the layers palette when she get the error message to see what the current Photoshop target is.
Other operations do change sizes of things. For example if you add or remove canvas the background layer size will change pixels will be added or removed from it. The other layers sizes remain the sane but their relative position over the canvas will be different. They may even seem to be cropped because the canvas size is acting like a clipping mask.
Selecting outer areas of a layer and clearing those pixels will change the layer bounds it will be cropped, be smaller.
Rastersizing an layer with a layer style like a drop shadow will add pixels to the layer content.
There may be more just think of what must be done to do something and ask you self did the number of pixels in the document change during an operation.