I have a sequence that has started to exhibit this behavior. No other sequence does this. I can extend a clip in the sequence to infinity, well beyond its contents. And every time I make a cut, I see this symbol.
The icon seems to indicate that it can join the clips at the edit point (though it hasn't shown up for me until now). I can still extend an audio track well past the end of the file, which means I can't verify sync with the end points. In the image above, the waveform stopping is the end of the recording.