Some time a file (usually an MS Word file) causes Studio to display the following error message:
Sometimes, Studio, instead of displaying the error (or in addition to it), shows garbage characters in the Editor:
According to the SDL knowledge base, this is likely due with problems displaying certain specific fonts (and, indeed, this source file did use some unusual fonts).
If this is the reason for the error (I believe it might appear in other instances), the way to overcome it, is to prevent studio to try to display formatting. You do this by going to the Options ( Tools > Options), and change the "Formatting display style" from "Show all formatting and tags"
To "Show all tags but do not show formatting":
When you return to the Editor you'll see that the text now displays correctly: