Ask HN: What database issues have you had, but only realised at a later date?

I have a web crawler which sometimes fails (e.g. the process dies, data comes in weird format, nulls etc). To make sure it's running properly, I'd really like an email when "something doesn't look right". So I'm thinking of building a tool to identify these anomalies / issues and make me aware of them.

So I was wondering...

What database issues have you ran into but only realised there was a problem at a later date? e.g.:

* The data pipeline died and data was no longer getting INSERTed

* The data format changed

* Started getting lots of nulls suddenly

* ...?