This is what it looks like when you have an internally contaminated rear group on your brand-spanking-new 18-50mm f2.8 lens. See the flare around the plane? It wasn't even that bright. In pix where, say, an LCD TV appeared in the image, it was blown completely out - haloed like mad, with complete loss of contrast anywhere near the TV screen.
The repair facility for Sigma doesn't stock groups, apparently. I have their assurance I should get it back in two to fourteen days. Way to narrow it down, eh?