Especially when they're speaking fast, use idiom, figures of speech or other things that can indeed be harder to translate than others, they often can't contain their incredulity in the interpreter's capabilities and feel they have to make that heard.
So often I've heard statements like "... I don't know if that was translated accurately..." (followed by a smile that clearly said "You couldn't possibly have done that correctly"), "I wonder how that came across in the translation" (sometimes followed by "if at all" - nice, speaker, charming - not...) - or worse, someone in the room who more or less speaks the language you're interpreting into decides to take over and give his own translation of something you have already accurately and fully translated yourself!
Colleagues may feel that this sounds familiar...
If you're colleague, and you're reading this - have you ever felt like hitting the person with the sardonic smile over the head with your microphone?
Have I? No comment... :-)