"A little knowledge is a dangerous thing" should be, "A little learning..."  (Pope — An essay on Criticism)

"Water, water everywhere and not a drop to drink" should be, "...nor any drop to drink". (Coleridge — 'The Ancient Mariner').
 
"Even tenor" should be, "They kept the noiseless tenor of their ways".
(Thomas Gray — 'Elegy Written in Country Churchyard'. NB In the same verse, the word 'madding' in the phrase "Far from the madding crowd's ignoble strife" means 'distracting' not 'maddening').

"In the sweat of thy brow" should be, "In the sweat of thy face shalt thou eat bread". (Genesis 3:19).

"To gild the lily" should be, "To gild refined gold, to paint the lily...is wasteful and ridiculous excess". (Shakespeare — Salisbury, in 'King John')

"A poor thing but mine own" should be, "An ill-favoured thing, sir, but mine own" (Shakespeare — Touchstone, in 'As You Like It', speaking of his girlfriend).

"Fresh fields and pastures new" should be, "Tomorrow to fresh woods and pastures new" (Milton — 'Lycidas')

"Rotten to the core" should be, "A goodly apple rotten at the heart". (Shakespeare — 'The Merchant of Venice').

"Escaped by the skin of my teeth" should be, "I am escaped with the skin of my teeth". (Job 19:20)

"Pride goeth before a fall" should be "Pride goeth before destruction and a haughty spirit before a fall". (Proverbs 16:18)

"Beware of Greeks bearing gifts" should be, "I fear the Danaans (Greeks), though their hands proffer gifts" (Virgil — 'Aeneid ii')

"A snake in the grass" should be, "A snake lurks in the grass". (Virgil — 'Eclogues, iii')

"Money is the root of all evil" should be, "The love of money is the root of all evil". (1Timothy 6:10)

"There's method in my madness" should be "Though this be madness, yet there is method in't" (Shakespeare — Polonius in 'Hamlet')

"Power corrupts" should be, "Power tends to corrupt, and absolute power corrupts absolutely". (Lord Acton).

"Blood, sweat, and tears" should be "Blood, toil, tears and sweat". (Sir Winston Churchill).