Grand unification predicted the universe should be littered with magnetic monopoles, and not one has ever been caught. Any grand unified theory, the class of models in which the strong and electroweak forces merge at energies near 10^16 GeV, produces topological defects when the unified symmetry breaks as the universe cools: pointlike knots of field carrying isolated magnetic charge, with masses around 10^17 GeV each. In the standard hot big bang the early universe necessarily passes through the GUT temperature, the phase transition necessarily occurs, and the Kibble mechanism necessarily produces roughly one monopole per causal horizon volume: enough relic monopoles to outweigh everything else in the universe by orders of magnitude. The observed abundance, after half a century of dedicated searches (induction experiments, MACRO, IceCube, cosmic-ray detectors), is zero.
Inflation's third job, after horizon and flatness, was janitorial: dilute the monopoles by stretching space until fewer than one remains in the observable volume. The fix works arithmetically and unravels conceptually, since it requires inflation to occur after or during the GUT transition with the reheating temperature kept safely below monopole-production energies, another tuning on the inflaton's ledger; and it leaves the deeper oddity untouched: the standard model of cosmology guarantees a GUT epoch whose sole observational consequence is a particle that must then be diluted into invisibility. The prediction and its erasure are packaged together.
The standing is a textbook problem with a textbook patch: monopole searches continue (and constrain), GUTs remain unconfirmed in every channel including proton decay, and a cosmology whose thermal history simply never reaches the GUT scale has nothing to clean up.