You might say this this person has gone a bridge too far.
An idiom inspired by Operation Market Garden, meaning an act of overreaching
This event also inspired a book:
A Bridge Too Far, a non-fiction book by Cornelius Ryan published in 1974, tells the story of Operation Market Garden, a failed Allied attempt to break through German lines at Arnhem across the river Rhine in the occupied Netherlands during World War II in September 1944. The title of the book comes from a comment made by British Lt. Gen. Frederick Browning, deputy commander of the First Allied Airborne Army, who told Field Marshal Bernard Montgomery before the operation, "I think we may be going a bridge too far."
And the book inspired a movie:
The name for the film comes from an unconfirmed comment attributed to British Lieutenant-General Frederick Browning, deputy commander of the First Allied Airborne Army, who told Field Marshal Bernard Montgomery, the operation's architect, before the operation: "I think we may be going a bridge too far."
Edit: You also might consider:
One rotten apple spoils the whole barrel:
A single bad influence can ruin what would otherwise remain good.
This can also be stated as:
- One rotten apple spoils the bunch.
- One rotten apple ruins the rest.
"He has a lot of good ideas, but his idea about free ponies is a rotten apple that spoils the rest."