Pain of handling 'Error condition' & designers' dilemma

We have have built a complete web analytics application (internal) for tracking Yahoo! products. Being the only designer for the product I got the chance to work on every BIT of it; from each and every component to every pixel. I can tell you its quite an experience handling such a big product on our own - that too when you are not only the interaction designer but also user researcher, product designer and visual designer. I'm not boosting but 'cribbing' :(

With so many flows and interconnections it become overwhelming. During design we fix these complex flows for most ideal conditions. The design works flawlessly; but then came the surprises - ERROR handling.

The product behavior was built such that the settings can be transferred. To give you an example, you are looking for mail data; then you put up certain segmentation settings like show PVs from India with age group 10-15 with users as male. Now you click on another metric like time spent - the rest of the setting of age, country & gender is transferred. The behavior is to change only the setting which user chooses to change.
This on paper looks flawless work well iin concept but then comes the conditions - in some cases it doesnt work. So this behavior works for most cases (lets say 90%) but in rest cases it throws up error.

Designers’ dilemma
Should I change the behavior to make sure system gives less errors (or conditions of no data found) or to provide a way which give most value?

Well, I choose option 2. Most use cases will work perfectly and give tremendous value, as the user settings are seamlessly transferred. I am providing value for 90% cases (this number is rough assumption) and spoiling the experience for rest 10%. But I guess its worth the risk; as it seems to be working :)

But then I had to figure out ways to make sure those 10% cases are also taken care of. This may not sound to be perfect, but we 'redirect' users to conditions that give data - of course with user consent. We show them the options of redirect.

That’s the best we could do for now.

But one thing I realized. Error conditions can some times spoil the party!
Your perfect design can be derailed with these fringe cases. But that what makes a complete design isn't it?