During fermentation, the guest bacteria produce lactic acid (which is what we're after) and release carbon dioxide gas. When it first begins, it is slow, but it picks up speed, and sometimes it proceeds so rapidly that the gas can't escape the fermenting mass quickly enough. If this happens, the mass can actually expand, displacing brine, and it can, in fact, fill up any headspace.
If you are using a ferment that has a fermentation lock, this can cause the brine to flow out into and through the fermentation lock! Under these conditions, the lid and lock must be removed, cleaned, and reassembled. Prudence dictates the use of a knife or spoon to help release any gas bubbles trapped in the fermenting mash before re-installing the lid, and it may be necessary to replace lost brine.
One must watch diligently for signs of mold. If the loading of the fermenter was completed expeditiously, and assuming it was properly assembled, there is unlikely to be any foreign growth other than Kahm yeast.
Kahm yeast, if it appears, will be a thin, often almost transparent, white film on the brine surface. This is harmless, but some contend that it may affect the flavor of the final product. Any that forms should be removed. If any objectionable odors, or obvious mold, or other growth of any kind, appears (especially growths that are black, or any color other than white!), safety dictates that the ferment must be discarded.