Oh - so many examples to choose from!
1)
We once did a validity check on a customer newsletter list. Dropped the number of valid e-mail addresses from 76,000 in half. Informed the internal customer, who asked us to hold off on deleting them until after the weekend, since they had already scheduled a party to celebrate the 75,000-subscriber milestone for Friday night...
2)
Consulting firm created a new flagship web site. We had required performance testing, which they did. During the big presentation they brought up the test and told the brass that no errors were encountered during the test. They were not very happy when I pointed out that a) the web servers were configured to respond with a redirect instead of an error code and b) more than a couple of dozen concurrent visitors would increase the page load times to double digits...
Already mad, they also mentioned that they had done usability testing and moved on with the meeting. I interjected and asked what the outcome of the UX-test had been. I already knew the answer, but it was fun to watch them try to spin "everyone hated it" into something less critical.
After that they made sure to keep me off the list of invitees.
3)
Then there was the time when we were looking for another hosting company. The Finance Dept, Legal guys, and the brass had already approved the contract when my manager gave me a copy of the response to the RFP.
That's when I discovered that they had listed some incidental sales under recurring income, thus inflating their annual revenue by $7M or so - doubling their actual numbers.
On my day off I sauntered into the office in my road bike gear to drop off the annotated proposal. eliciting the expected stares. My manager took one look at my notes and escalated it to the VP-level. They swiftly cancelled the signing ceremony with the vendor, which had been planned for noontime that very day...
A couple of months after that I found myself at some Microsoft training, sitting behind three lead engineers of this vendor. It was a wee bit awkward, to say the least...