We kept a plaintext file of hostnames, one per line, and a monitoring script read the file and pinged each host every five minutes. When a host failed to respond, the script sent an email alert. The system had been running for months and it worked — we had caught three actual outages with it, which gave us real confidence in the setup.
app-07 was added to the list on a Thursday afternoon. The engineer who added it was using VS Code on a Mac, and VS Code by default does not add a trailing newline to a file when you append to it using certain editing workflows. The file had ended in a newline before the edit. After the edit, the last line — app-07 — had no trailing newline.
app-07 went down the following Sunday afternoon at 2:17pm. The monitoring script ran at 2:20, 2:25, 2:30, all the way through the evening. No alert ever fired. The on-call engineer found out at 8pm when a client emailed. The system had been down for almost six hours.
When I looked at the script, the bug was immediately obvious once I knew what to look for. But I had written that script, I had tested it, and I had been looking at the monitoring confirmation emails for months without ever noticing. The confirmation email listed the hosts it checked. app-07 was never in the list. I had been reading those emails without actually counting the hosts. I just scanned for the OK lines and moved on.
