A nice UX ROI tidbit from Karl Sabino over on the Think blog:

Once the analytics were up and running, we could quickly see which pages occurred before and after the error page. This let us identify the user journeys where the errors were triggered. So we were able to write custom messages, things like “We’re sorry, we’ve had a problem processing your order. Your card hasn’t been charged yet. Please click checkout to try again.” We also provided a customer care number together with a code for continuing the transaction offline.

[…] Within a month the percentage of completed purchase journeys increased a modest 0.5%. Putting it in some perspective that 0.5% was worth £27,000 a month on average – or over £250,000 per year. And all this was hypothesised, diagnosed and implemented over 2 weeks, at a staff cost of approximately £8,000. That’s a great return on investment from paying attention to the detail of the customer experience.

Read Karl’s full article: £250,000 from better error messages