Science fiction has a pretty good track record of telling us what kinds of technologies (submarines, credit cards, cellphones) we should be expecting years, sometimes many decades, before they actually show up. But some technologies never make it out of our imaginations.

Tell us in the comments about the science fiction technology that you never expect to make the jump into the real world, and also tell us why (Too complex? Too dangerous? Too expensive to get right?) you think the case is so hopeless.