In 12 years of using Photoshop I've never seen an issue like this.
I have a picture. When I choose Vɪᴇᴡ ▶ Aᴄᴛᴜᴀʟ Pɪxᴇʟs the image is displaying incorrectly on my monitor. On the left is what my image looks like (zoomed up by a factor of 4 using nearest-neighbor). On the right is a screenshot of what it looks like when displayed at "Actual Size" (also zoomed up).
What setting could possibly be causing this?
Photoshop CS4 on Windows 7x64, on a machine with multiple graphics cards by NVIDIA.