Headphones confused with Handsfree

[o2bnclemson]

From time to time, my Eris seems to confuse my headphones with a hands free device. As a result, if someone calls while it thinks it's a hands-free then I can't communicate with the caller like I can when it thinks it's just a headphone attached. It's easy to tell but I've no idea what triggers it - it's either a headphone icon on the status bar or a headphone with a mouthpiece attached indicating a hands-free device.

Anybody know what's up with that?