We hit a bug in Canvas text handling recently, where Speedgrader was unable to render a Submission Comments PDF for several submissions. It turned out that because one of the comments had a strange symbol in, it broke the PDF generator (error code from the case we logged - Your document includes text that's not compatible with the Windows-1252 character set).
The Canvas Assignment Comments field needs to be more tolerant to weird symbols, by automatically throwing them away. We had a plugin on our moodle which, on detecting a Paste attempt, would offer the user the chance to choose whether to keep formatting or just paste the text plain (e.g. how Paste as plain text works in Chrome when you right-click a text entry box). We will be telling users to use that right-click as a workaround, but really the core software should be doing this automatically. Or at least give users a choice, where we tell them to not use that choice.