When errors reach a certain threshold, an alert of some type (HTTP endpoint, email, SMS, whatever) is generated.
Maybe when errors:
1) Errors per time period reached a certain threshold (100 errors / hour == alert)
2) Errors increased a certain amount per time period (error rate 100% greater now than an hour ago)
I would like Raygun to let me know when my application starts generating an uncharacteristic amount of errors.