Wow.  I can see myself making a mistake like that, even though signing returns implies that we have reviewed them for errors.    If you can't trust software to add two numbers, can you trust your calculator?  And eyewitness testimony is always the least reliable.  

For federal returns I try to use the two-year comparison feature to spot anomalies.  But that's not available for most states, right?  

I would look for other joint returns filed that week.