How do you test if a process has too many edge cases before you automate it?

We built an automated script to handle a tedious weekly task and thought we saved a ton of hours.

Instead tiny bugs started slipping through without any alerts.

Now the whole team has to go back and double-check every single piece of output by hand just to be safe.

What’s your rule of thumb for deciding if a process is too unpredictable to automate?