Me: Sure, we'll be ready to parse and load those data files on 2005-02-07. Sounds simple. No problem. We'll just need some things from you, first. Can we get a sample of the data file?
X: Well, actually we won't have any of those until the 8th. The system gets integrated on the evening of the 7th, and then we begin generating data.
Me: Well, we have your vague description of what the file will be like from which we can sort of start writing the program, but we can't test until we have test data.
My requirements manager, to X: Couldn't you please find some way to generate a minimal test file, so we could have an example to work from?
X: Well, we could provide some of that, but when the system goes live, it could be very different. I'd hate to have you guys do all that work and then find out the files we sent you don't match the real files in production.
Me (dryly): Well, that's exactly the situation we'll be in if we do the work with no files at all, right?
Notice how the time cost of my effort is assumed to be zero, except when it's convenient.