Accidents will happen
Things sometimes go wrong, despite our best efforts.
This proverb has been overtaken by the more vulgar 20th century 'shit happens'. The original has its origins in the 18th century, as in John Muller's A treatise containing the practical part of fortification, 1755:
But, notwithstanding all human precautions that can be taken, yet accidents will happen.
Charles Dickens took up the thought a century or so later, with this the most well known expression of the proverb, in The personal history of David Copperfield, 1850:
'My dear friend Copperfield,' said Mr. Micawber, 'accidents will occur in the best-regulated families; and in families not regulated by that pervading influence which sanctifies while it enhances the - a - I would say, in short, by the influence of Woman, in the lofty character of Wife, they may be expected with confidence, and must be borne with philosophy.