I said yes, I had 1 other interview coming up (in reality I had 3 but was afraid to inform him). Since he seemed like he was interested and was about to offer me the job, he was requiring an answer in 3 days; My interviews were taking place over the next week. I did get an offer but then he needed t think about it. In the end, I did not get the job.
That is actually pretty common. You have to think of it from the Employers point of view; they have a list of potential employees. They start with #1, then move down the list as needed. So in order to be sure they fill the position best, they ask that you let them know either on the spot or within a few days so they can send out another offer... Sucks for the interviewer, but its business sense from the employer.