There are special characters that can cause problems in word press, like the infamous microsoft word smart quotes. However, there are many that are captured and dealt with. My collegue and I have written a plugin that, even if it just passes the content through, stops wordpress from handling many of these characters properly. It shows particularly on foreign language web sites like a German one where special characters are handled just fine until my plugin is activated. Why is this so?
Let's give this a guess out of the blue: Check the encoding of your plugin file. It should be ASCII, also named US-ASCII or ascii-7bit. This ensures that your plugin is most compatible to wordpress installations out there.
This information is probably useful for you to find out about the actual encoding of your plugins files (you need to encode all as ascii 7 bit if you have more then one): How Can I Determine the Encoding of a File?.
To change the file encoding, this depends on the editor you are using. As I don't know which one, I can not offer any more information about this then, that editors tend to have an option to specify the file encoding in the save-as dialog -or- somewhere in the menus while you have opened the file in question.
If you have characters in your file the get broken when you save your file in another encoding (make a backup first!), you might probably want to encode them as UTF-8. That done, the file is still compatible with most (but not all) sites. And this might prevent you from loosing characters in your file(s).