Black tabs

[Giordano Bruno]

I've never seen this before and my internet search suggests that it has only ever happened to me. My workbook has five worksheets. If I save the workbook when the last sheet is active, and then put the cursor over another tab, the tab turns black and every tab that the cursor runs over turns black (not the last sheet though). Clicking the other tabs doesn't change the active worksheet. If I use (Ctrl-Page Up), I can change tabs and the black backgrounds go away. If anyone has any suggestions I would be very grateful to hear them.