Small issue, big problem?

One thing I've learned over the years is that the stuff that causes the biggest headaches usually doesn't look important at the beginning.

A tiny detail gets overlooked, everyone moves on, and then weeks or months later you're stuck dealing with the fallout.

It made me curious about other people's experiences.

What's a situation where something that seemed minor at first ended up becoming a much bigger problem than expected?

And what did you learn from it?