Open three workbooks in one instance of Excel, say Book1.xlsx, Book2.xlsx and Book3.xlsx.
In Excel 2010 and earlier, the Ctrl+Tab and Ctrl+F6 keyboard shortcuts would switch between all of these workbooks. For example, with Book1 selected, Ctrl+Tab takes you to Book2. Pressing it again takes you to Book3. Pressing it again takes you back to Book1.
In Excel 2013, the shortcuts on;y select the first two workbooks. For example, with Book1 selected, Ctrl+Tab takes you to Book2. Pressing it again takes you back to Book1. There is no way to switch to Book3 with one keystroke.
Workarounds
Use Alt-Tab. This is annoying because I have to navigate through all the applications and windows that are open.
Use the keyboard shortcut Alt+W then W then <number>. This relies on remembering which number refers to which workbook.
Use Ctrl+Shift+Tab. I tried this at the suggestion of another trainer here and was surprised that it works.
Notes
This is not an issue with Word 2013 or Visio 2013. Ctrl+F6 is switches correctly in both; Ctrl+Tab switches correctly in Visio (in Word, Ctrl+Tab is used for something else).
Version
Microsoft Office Professional Plus 2013, the NFR copy from my TechNet subscription. 32-bit version of Office running in the 64 bit version of Windows 8 Pro.