Is there a limit to how many characters can be added to a single comment?
Yes, the character limit on ticket comments is 65,535. For example, if you submit a plain text comment with 100,000 characters, it will always be truncated to the first 65,535.
If markdown or rich text formatting is enabled and in use, comments may be truncated even more. Extra characters are needed to encode any kind of formatting used in markdown and rich text formatting. Even if these characters are hidden, they still are counted towards the character limit.
This is considered expected behavior, and at this time there is no precise warning that a comment may be truncated if it goes over this limit. There is an ongoing post in our Product Feedback forum about this issue here: Do NOT Truncate Comments Without Warning.