It used to be that almost all projects I worked on dealt with only one database or one file format. But that was before I started working on integrations in the retail sector. With a Point-of-Sale system from one vendor, shipping software from another vendor, exports to an accounting system, and all synchronized to an online shopping cart, things can get pretty crazy. Whether importing XML files in one end and writing to a database on the other, or calling a web service on one end and uploading a file to a shared folder on the other, everything has to be timed and monitored. I've seen the smallest change cause a cascading chain of missing data and grumpy retailers.
In addition to the questions asked in standard projects, there can be many more moving parts to understand. How often does the web site need to be updated? Are there different prices shown at one location than another? How do customers use their store credits when making a purchase online? How do you tell what items have been changed or sold so you can keep data packets small? What about address verification or shipping rates or coupon codes or multiple time zones or different tax areas or gift wrapping?
Every time I think I've seen it all, I am handed a project with a new twist. But that's the fun of it, right? A good challenge to keep me on my toes!