The ICSharpCode.TextEditor has a minor bug in it's Copy functionality when the control is used with a document with syntax highlighting and with a font size that isn't evenly divisible by 0.5. I'm not sure if this situation ever occurs in the SharpDevelop application, but it's fairly easy to reproduce if you reuse the TextEditor control in any other Windows Forms application, especially where Visual Studio chooses 8.25pt as the default font and size.
When copying from the control under this situation, the plain text inserted onto the clipboard is correct, but the RTF text is incorrect, and will manifest itself by appearing to append extra text to the beginning of the copied text when pasted into an editor that supports RTF text, such as Word. The extra text may appear to be ".5 " or similar.
The problem does not occur when the document in the TextEditor control does not have a highlighting strategy, as no RTF representation of the text is generated in that case.
Digging into the code, the problem occurs in ICSharpCode.TextEditor.Util.RtfWriter's BuildFileContent method (at line 132 in the 3.2.1.6466 source). The line is below:
rtf.Append(@"\f0\fs" + (textArea.TextEditorProperties.Font.Size * 2));
Per the RTF spec, the "\fs" command can only be followed by an integer; however when given 8.25pt as a font size, for example, this code would insert "\fs16.5" into the RTF stream, which is actually parsed as "\fs16" to set the font size to 8pt, followed by ".5" as text.
Correcting the problem is as simple as sending the result of the *2 multiplication through Math.Round:
rtf.Append(@"\f0\fs" + Math.Round(textArea.TextEditorProperties.Font.Size * 2));
With this change, the copied RTF text's font size may be off by up to 0.5pt, but at least the RTF's text will accurately mirror the text that was copied from the contol.