Speakerphones and similar not-easily-visible communications devices provide endless opportunities for PCs to say something when they're not supposed to.

Of course any meeting with a high-powered business executive has to have someone inconvenient listening in.

But imagine storming into the castle and demanding an audience with the king, bursting into the throne room and blurting out that his cousin is planning a plot to overthrow him and claim the crown for himself, only to have said cousin speak up from a previously unnoticed magical communication device, saying, "I told you you can't trust these lying so-called heroes!".

Transcript

Darth Vader: Governor Tàrkin, this is no random attack. For safety, I formally urge you and the Senate to evacuate the Peace Moon.
Tàrkin: Evacuate? In our moment de triomphe? Ah think you overestimate zeir shonces.
{General Bast approaches}
General Bast: Governor Tàrkin, the Moon Ghost is taking over all our systems!
Tàrkin: Snap out of it, Zheneral! Zere is no such thing as ze Moon Ghost.
General Bast: We're doomed. We're all going to die.
Darth Vader: Moon Ghost, General Bast?
General Bast: Lord Vader is on the line? I mean... everything is working perfectly!
General Bast: {sotto voce} And... I'm definitely doomed.