17:12 UTC, there was a unreported network issue. It caused two of our reverse proxies to fail.
17:13 UTC, two alerts get sent through the on-call system. The on-call person ACK both of them, handles the first one and mistake the second one for a redundant alert of the first one.
18:30 UTC, some customers complain about issues between APIs. We start investigating.
19:45 UTC, the culprit is found: a reverse proxy was down. It is restarted and everything goes back to normal.
19:50 UTC, we find the unattended alert and understand the mistake that was made. (reading the two alerts as one issue.)