Considering Magic: The Gathering, for a totally random group of cards covering the full-range of *Magic* (all the way back to 1993), the machine may miss 1 out of 1000, so 99.9% accuracy, but there are a couple of caveats to note:
1. The cards need to be in our database. We haven't fully integrated every set of cards, and with Wizards continuing to splinter each set into increasingly intricate sub-sets, this is a constant battle. Generally speaking, we have everything loaded, but there are some edge cases that we're still working our way around too (mostly special promotional cards).
2. For cards that are reprinted in multiple sets using the same artwork, and where the set symbols are very similar (the Core sets, for example), it is possible the set will be misattributed. We are always improving our set recognition algorithms though, and eventually we should be able to eliminate set misattribution entirely. As a work-around, we've made it very easy to change the set of a card that has reprinted artwork in our GUI.