Worst mistake I have made is accidentally wiping a database table by messing up a SQL script. This was caught after it had been deployed to around 30 customer sites, most of which had no technical staff on site, and we didn't have remote access to. I bought our support team cookies for fixing that one.

3 days from planned go live date and the client turns around and says "We need these features added before we can go live". New features pretty much double the existing workflow, and require extensive rework. Client is insisting on meeting the original date, despite the product being "not usable" until the changes are made.