What Are the Most Common Reasons for Unexpected AWS Cost Increases?

I’m curious to hear how others manage AWS costs as their infrastructure grows.

What have you found to be the biggest causes of unexpected AWS bills? For example, unused EC2 instances, oversized resources, storage, data transfer, or something else?

Also, what tools or practices do you use to regularly monitor and control cloud spending?

Would be interested to hear real-world experiences and what has worked for you.