In spaceflight, an anomaly is an anomalous outcome compared to what was expected. The severity of the anomaly varies. Typically, if the world outside of the organization hears about an anomaly, it was severe enough to cause a Loss of Mission (LOM) or Loss of Vehicle (LOV). Internally, when things behave anomalously, they're referred to as off-nominal, and are subject to internal investigations to determine the cause.

This is a _very_ off-nominal outcome and the investigation will absolutely involve outside organizations and halting the program during the investigation until the investigation completes with a sufficient determination of faults and accompanying remediation plans.

It normally doesn't explode of course!

The linked tweet literally says "it blew up", though. "Anomaly" is just a word used in rocket science lingo that makes for a funnier headline.

The space industry people have been saying this since 1997: https://youtu.be/z_aHEit-SqA?si=N6-PtezdsOhsv63O

Delta 2 rocket exploded during launch, raining flaming debris everywhere and the announcer says we had an anomaly

It was slightly more fiery than a mere "observation", hence "anomaly".

In engineering, there is a preference for accurate terminology

It underwent rapid unscheduled disassembly.

The explosion was an anomaly.