Ere is a preposition, often seen in old poetry, meaning before or previous to. Err is a verb meaning (1) to make an error or a mistake, or (2) to violate accepted standards. It is the word used in common expressions such as “to err is human” and “err on the side of caution.”
Examples
Ere sounds archaic, but modern writers sometimes use it to give their text a poetic ring—for example:
Summer will arrive ere long. [The Telegram]
Ere she was installed as the czarina of Writers’ Building, Mamata made known her demand. [Economic Times]
Ere we had time to process this unwelcome datum, we landed hard on something soft, which gave out a “Whoof” and a “Squeak.” [Northfield News]
Err is more common. It’s usually used in the phrases mentioned above, but it’s useful elsewhere:
Liberal pundits err when they assume a figure such as Santorum is there to propound a governing agenda. [Washington Post]
[Y]ou err, confess, make penance, receive absolution, err again, confess again. [The Road Washes Out in Spring, Baron Wormser]
The opposition on Wednesday slammed the administration for treating erring local officials from the ruling Liberal Party with kid gloves. [Manila Standard Today]