It is not the 'escape' that must be foreseeable, but only that the 'damage' was foreseeable, if the 'thing' does escape
Examples of 'things' which the courts have decided can do mischief: Gas and electricity, Poisonous fumes, A flag pole, Tree branches, An occupied chair from a fair-ground ride (Hale v Jennings Bros 1938)
Shiffman v The Grand Priory of St John [1936] - A flag pole belonging to the defendant fell and hit the claimant. This was held to amount to an escape for the purposes of Rylands v Fletcher. The defendant was made to pay damages as compensation.
The 'thing' must escape from one property onto another (however, not always strictly applied - Jennings for example, both stalls at the fair operated on the same piece of land)
Read v Lyons (1947) - An inspector was checking the interior of munitions factory and was injured, together with a number of other workers, when a shell exploded. The House of Lords (Now Supreme Court) decided that Rylands v Fletcher did not apply as 'there was no escape at all of the relevant kind'
Viscount Simon in Rylands v Fletcher stated "an escape from a place where the defendant has occupation or control over land to a place which is outside his occupation or control"
Escape from land or circumstances where defendant has lost control
British Celanese v AH Hunt Ltd (1969) - an escape where the defendant has lost control. The defendants stored strips of metal foil used in the manufacture of electrical components. Some strips blew off the defendant's land onto a electric sub-station, causing power failure. The defendant had lost control of his land, but it was held that the use of land by the defendant's was a natural use of land, because of the benefit from the manufacture received by the local population.
Act of God - may succeed where there are extreme weather conditions that ‘no human foresight can provide against’. (no one would have foreseen such weather).
Act of a stranger - if a stranger over whom the defendant has no control has been the cause of the escape causing the damage, then the defendant may not be liable.