I've done that a few times. Mostly when the previous bag was used the night before, and I was super sleepy in the morning, so didn't even bother ditching it, saving 1.3 seconds and thinking it would make my new tea stronger.

...yeah, I don't do that anymore. But this is why I used to.

UPDATE: I just made my tea just the regular way this morning. While stirring, I realised I had left the previous night's red berry tea bag in it. I didn't want to waste an otherwise perfectly fine bag of Earl Grey, so I did it again. Not intentionally, though. Also, note to self: red berry Earl Grey is not great.