SF/Fantasy is an extremely broad, mixed sub-genre of SF drawing elements from both science fiction and fantasy. Although in some terms of its portrayal in recent media products it can be defined as instead of being a mixed genre of science fiction and fantasy it is instead a mixing of the genres science fiction and supernatural.

Sci-Fi describes unlikely things that could possibly take place in the real world under certain conditions, while SF/Fantasy gives a veneer of realism to things that simply could not happen in the real world under any circumstances. Another interpretation is that Sci-Fi does not permit the existence of fantasy or supernatural elements whereas SF/Fantasy does. Even the usage of this definition is difficult, however, as some SF makes use of apparently supernatural elements such as telepathy although science fiction can use telepathy or telekinesis because they deal with the mind and such abilities can be accessed or created through scientific means.

Some SF/Fantasy stories might contain space travel, laser weapons, alien species, and magic.

Thief of Time by Sir Terry Pratchett is a superb example of this particular sub-genre. 

The Auditors are upset because the human race (although this appears to include all races on the Discworld, not just the humans) are living their lives in - what the Auditors consider to be - an unpredictable (and therefore not understandable) way. To fix this matter once and for all, they decide to convince a young clock maker, Jeremy Clockson, in Ankh-Morpork to build a perfect glass clock. They do not reveal that this will imprison Time (the anthropomorphic personification) and thereby freeze time (the physical quantity) on the Discworld. By freezing time, the Auditors intend to eliminate the unpredictability that humans cause through their everyday actions (and have enough time, for once, to file all the paperwork). Death discovers their plans, but is unable to act directly because of previous agreements with the Auditors. Instead, Death sends his granddaughter Susan to stop them, assisted (apparently) by the Death of Rats and Quoth the Raven.

Meanwhile, in a distant valley, a young apprentice of the History Monks, Lobsang Ludd, is apprenticed to Lu-Tze, known throughout the Oi Dong Monastery as 'The Sweeper'. Following a highly impressive patch-up job with time (the physical quantity) by Lobsang, he and Lu-Tze are brought before the Abbot, where they hear that a glass clock is being built. Lu-Tze knows of such a clock's side-effects, since he was sent to prevent a previous clock from being built in Überwald. However, due to the difficulties inherent in Überwald (including the difficulty in determining which specific bolt of lightning hitting which castle might start the glass clock) he failed to stop the original - but Time only froze for a moment before a metal spring snapped and caused the clock to shatter. Having figured out that the new glass clock would be likely built in Ankh-Morpork, Lu-Tze and Lobsang head for the city to try to stop Jeremy from building it.

From that brief outline, one can see many elements of both SF and Fantasy. There are many other such examples within Pratchett's Discworld series.