By contrast, it surprises me when the electronic resource I use looks the same from one day and the next. Even CD-ROMs, which would seem to be a fairly permanent format, vary when they are replaced with an updated disc or have an upgraded interface. This creates some thorny issues when one imagines cataloging these resources. If the title on the CD-ROM stays the same, but the time period covered by the disc or the contents themselves change with each update disc, this seems different to me than a series of periodical issues that each cover a fixed unit of time. Even if this were the case, it is very difficult to imagine the process by which a cataloger would discover the changes that need to be included.
Electrons differ from printed materials in the ways they arrive at the library as well.
They tend not to come in a truck, since electrons have only a negligible physical presence. When they are not being used, they exist in a state of potential, rather than reality. The particular reality that exists for any user will very likely differ in appearance and capabilities depending on the size of the pipe and the type of faucet being used to access the resource. The version one finds on the World Wide Web may differ in substantive as well as graphical ways from the version reached through telnet. To put it another way, one may not know a book by its cover, but at least it has one.