[GENERAL] Error: "cached plan must not change result type"

At Friday one of our clients got this error:

    "cached plan must not change result type"

He restarted the application and the problem vanished.

We used PGDAC to access the database.

Firstly we didn't know nothing about this kind of error.

But later we realized that somebody in our company altered a very often used table. 

He added only one field with default value while they were using the programme (they didn't exit).

Just for we will be sure: these modifications could generate this kind of error?

What is the complete description of this error? What we need to not do for avoid it?

Thank you!

Best Regards

   dd