Once upon a site migration...
...I was tasked with getting about 10,000 pages of website from our old CMS to our new one. Having spent several months poking around in the piles of spaghetti code that tied together a few hundred modules that had been attached to the old site in order to get it to do the dozens of intranet things that it didn't really have any business doing, I came to the conclusion that the fastest way to get everything moved would be to get each page owner - and helpfully the pages had the owners name stamped on them - to simply copy-paste the pages in question into the new site and do some much needed update on the copy as they went along.
I was told in no uncertain terms that there was no way everyone had time for that, and we had to automate it. I said something along the lines of "OK, but I'm giving this a 50% chance of working at best, and given the number of interactions we might not find out exactly how bits of it are broken until it's too late to do anything about it.
It was decided by the website committee* that we would go ahead with the automated migration, and so one fateful morning I pressed the enter button on an utterly terrifying bash script calling various bits of perl, php, and in one instance piping an SQLdump through awk. It was not pretty.
It took about an hour to realize that it was taking much longer than it ought to and clearly something somewhere had gone wrong and we were going to have to revert to backup even if it did finally complete, and I informed the website committee of this. Again, the committee decided* that it would be better to wait for it to finish and then spend a couple of days cleaning up the errors on the new site than going back and doing it all again. I pointed out that this was a very bad idea since I didn't really understand how it was that it had broken in the first place because the code I'd written couldn't possibly cover every weird edge case that I suspected, but couldn't prove, existed.
Three weeks later everyone agreed that we needed to go back to the backup and start over.
*That is, no one is prepared to take actual responsibility for the decision, and the only person who clearly opposed it** was mysteriously omitted from the minutes.
**Me.