There were strange rubber plugs in the braze-ons for a second bottle cage on my bike. They were pretty much completely melded with the frame (I got it from my boss, who said it had been sitting in her garage for about 10 years) so I cut off the caps with a knife, and then poked the stems through so they fell into the tube of the frame.

Now when my friends ask me what that strange pinging is when I ride I ask "What pinging?"

I suppose someday I won't be lazy and will actually take the seatpost off and shake my bicycle upside down until the stems come out, but I'm not seeing it happening in the near future.