(Using Simone's 2.8.6 build on Mavericks.)
Pasting an image into Gimp worked the first time, but after I did a bit of copying and pasting within Gimp then pasting an external image in didn't work anymore.
- Copy an image to the clipboard from somewhere else, e.g. by taking a screenshot.
- Paste the clipboard into Gimp to start a new image (File > Create > From Clipboard).
- In Gimp, do some copies and pastes within the image.
- From another app, copy an image to the clipboard (e.g. take a screenshot).
- Attempt to paste the clipboard into your Gimp image. At this point it doesn't work, instead you get the last internal image from step 3 repeated.