The difficulty here is that a crack can be in different places and into engine function areas or a harmless crack.
Water jacket - loss of coolant into the sump or cylinder and possible bubbles into coolant and or overflow of the cooling system.
Into an oil gallery - loss of oil pressure if internal or leak outside the engine if external.
Into a cylhead bolt hole - may cause a problem torquing the affected cyl head bolt or weaken the cylhead gasket seal.
In an internal web- may cause crankshaft flex or vibration at high rpm may eventually cause block failure - fairly rare.
Block sump flange - oil leaks. Not common.
Water jacket to external place - evidence of coolant stains on the block and loss of coolant - not common but does happen.
All of the above represent a faulty block which often does not show up while the engine is new as chemical treatment of the block along with painting often temporarily blocks the crack. Having said that cracked blocks are not that common but some specific brands/models are prone due mostly to something overlooked in terms of cooling the block when cast.