I've heard that can happen with a corrupted font (though I've never experienced it).
Does this happen with one file or more than one file? If one file, the file could be corrupted. Save as IDML and reopen in InDesign.
10.11 is not Yosemite…it’s El Capitan.
CS6 is not supported and this is an oft reported problem. Roll back to your earlier operating system or upgrade to Creative Cloud.
CS6 has reached end of life and will not be updated.
It happens very often with different file types. Unfortunately not always with the same character. Difficult to determine which character might be. I have so many assets to work. Thanks anyway
Missed the OS X version number reference, thanks.
I'd agree it's definitely the incompatibility of El Capitan and CS6. Happens very regularly.
thank you so muchGianmarco