The “Review of the Safety of LHC Collisions” by the LHC Safety Assessment Group, 2008, (see references below) studied the possible production at the LHC of hypothetical objects such as vacuum bubbles, magnetic monopoles, microscopic black holes and strangelets, and found no associated risks.
Microscopic Black Holes
Regarding microscopic black holes they say:
As has already been discussed, the LHC will make collisions with a much lower centre-of-mass energy than some of the cosmic rays that have been bombarding the Earth and other astronomical bodies for billions of years. We estimate that, over the history of the Universe, Nature has carried out the equivalent of 1031 LHC projects (defined by the integrated luminosity for cosmic-ray collisions at a centre-of-mass energy of 14 TeV or more), and continues to do so at the rate of over 1013 per second, via the collisions of energetic cosmic rays with different astronomical bodies.
In other words, nature is already conducting far more experiments with far more energy than the LHC, with no adverse consequences. In addition:
[A]ccording to the conventional gravitational theory of General Relativity proposed by Einstein, many of whose predictions have subsequently been verified, there is no chance that any black holes could be produced at the LHC, since the conventional gravitational forces between fundamental particles are too weak.
So black holes cannot be produced, but even if they did, the report says there would be no adverse consequences.
On the other hand, they never considered the possibility that a black hole might be produced if a cosmic magnetic monopole were captured by the LHC or one of the detectors. Hence there are, perhaps, more questions raised than answers.
Regarding magnetic monopoles, the report says
[I]f monopoles could be produced by the LHC, high-energy cosmic rays would already have created many of them when striking the Earth and other astronomical bodies. Since they would have large magnetic charges, any monopoles produced by cosmic rays would have been stopped by the material of the Earth . The continued existence of the Earth and other astronomical bodies after billions of years of high-energy cosmic-ray bombardment means that any monopoles produced could not catalyze proton decay at any appreciable rate. If the collisions made in the LHC could produce dangerous monopoles, high-energy cosmic rays would already have done so.
Note  refers to “Astrophysical implications of hypothetical stable TeV-scale black holes”, which can be downloaded below.
Capture of a Cosmic Monopole
“Astrophysical implications of hypothetical stable TeV-scale black holes” says that
The stopping power for monopoles moving through matter is about 100 times larger than that of a charged particle, since their magnetic charge is at least 1/α times greater than the electron electric charge. This means that the Earth itself can stop magnetic monopoles of masses much larger than those that can be produced at the LHC.
However they do not consider the possibility that a massive monopole, produced by the adsorption of hadrons within a star, might be captured by the magnetic field of one of the detectors, as is postulated in “Time Crystal”.
Wyken Seagrave and Penny Press have no hesitation in endorsing the conclusions of the team of international scientists which has reviewed the safety of the LHC and found it completely safe. Time Crystal is, after all, a piece of fiction. However, we would argue that the idea proposed therein, that a cosmic monopole becomes super-heavy by adsorption of hadrons so that it is not destroyed by passage through the outer layers of the Earth, is as sufficiently feasible to make the story credible, and allow the reader to suspend his disbelief, which is all that a work of fiction requires.
Review of the Safety of LHC Collisions can be downloaded here: LSAG-Report
Astrophysical implications of hypothetical stable TeV-scale black holes can be downloaded here: Astrophysical Implications
The Safety of the LHC, CERN Press Office Statement.