Mistake

Robert Browning published "Pippa Passes", a dramatic piece, in 1841.

The song

The year's at the spring,
And day's at the morn;
Morning's at seven;
The hill-side's dew-pearled;
The lark's on the wing;
The snail's on the thorn;
God's in his Heaven -
All's right with the world!

is well known. Less well known is

But at night, brother Howlet, far over the woods,
Toll the world to thy chantry;
Sing to the bats’ sleek sisterhoods
Full complines with gallantry:
Then, owls and bats, cowls and twats,
Monks and nuns, in a cloister’s moods,
Adjourn to the oak-stump pantry!

Twats? Browning explained later that he had read the lines

They talk’t of his having a Cardinall’s Hat;
They’d send him as soon an Old Nun’s Twat.

in a poem "Vanity of Vanities" from 1660, and assumed that a 'twat' was an item of nuns' headgear.

Well, we all make mistakes. Such was the purity of the Victorian mind that the error supposedly went unnoticed for forty years, until an OED editor questioned him about it.