In Parashat Miketz, Yosef's brothers encounter him disguised as the viceroy of Egypt, and he accuses them of espionage, jails them for three days, and makes a demand that they know their father would never allow. In his presence, they discuss their suspicion that they are being Divinely punished for their mistreatment of Yosef years earlier. They have no reservations discussing this in front of Yosef because (Gen. 42:23):
וְהֵם לֹא יָדְעוּ כִּי שֹׁמֵעַ יוֹסֵף כִּי הַמֵּלִיץ בֵּינֹתָם
They did not know that Joseph understood, for the interpreter was between them.
In other words (according to Rashi's intuitive explanation), during their entire encounter with their disguised brother, they'd been speaking Hebrew to an interpreter, who was translating into Egyptian for Yosef. So, they assumed that Yosef didn't know Hebrew and that they could therefore speak freely in this language.
I've always wondered why they weren't concerned that the translator himself would report their conversation to Yosef. Does the verse really mean that the interpreter had been between them, and now he was on a pistachio break? Or, was there some sacrosanct code of interpreters that they would never report on overheard conversations? If Yosef had Hebrew interpreters at his command, shouldn't they have suspected that he'd have Hebrew-understanding spies listening to their conversations also?