I have observed this problem in 2.5.0 and it still occurs in 2.5.1b1.
It only happens, if I copy text from Word or Excel: When you paste the text, it looks like you had tried to paste an image.
My suspicion is, this happens due to these Microsoft Office programs putting the selection into the clipboard in the CF_DIB (device independent bitmap) format, not only as text, Unicode and Rich Text. So Spark think, you are pasting an image and those are supposed to be sent right away.
Screenshot of some cells from Excel pasted into Spark:
Who files a bug?