Originally Posted by
Rocky Gravol
The broken link, and the link that almost broke.
This looks like a modern, riveted chain that requires a master link to re-join the ends when opened. Driving the rivet out with a chain tool destroys the peening that holds the link together. If you re-join the ends without a master link, the damaged rivet fails to hold the link reliably. I suspect this might be what happened to you.