I just wrapped up a project with a client extremely concerned about tricksters (different from hackers). The site we created allows users to sign up for special offers to be sent via cell phone. The client repeatedly brought up the worry that a trickster would enroll other people in the program, thereby harassing them with a text message that prompts a double opt in. Despite our concerns that this scenario wasnt very probable, the client pushed back enough times that we eventually added in quite a bit of functionality to prevent this, some of which we believe mildly damaged the more typical user experience in the process.
Now that the project is complete, Im wondering if others have encountered client concerns about tricksters or have their own concerns. Any stats out there on the likelihood of a site being abused? I know blogging sites are vulnerable to spam, for instance, so many have those warped letter entry fields. But our site involved a 2-page, rather lengthy signup (including supplying a phone number and creating a password). Anyone ever seen a signup form like that misused?