an Achilles' heel

an Achilles' heel

a small fault in a person or system which might cause them to fail
Usage notes: Achilles was a man in Greek mythology (= an ancient set of stories) who was killed when he was injured on the heel. This was the only part of his body where he could be harmed.
As a team they're strong on attack but they have a weak defence that might prove to be their Achilles' heel. Vanity was his Achilles heel.
See also: heel
References in periodicals archive ?
I worry and wonder if we have built an Achilles' heel into the system.
The loop is "like an Achilles' heel," says Fernandez.
Is that going to be an Achilles' heel through the tournament for the United States?
I had an Achilles' heel - I couldn't hit it very straight.
Search engines are an essential tool in the workplace, yet they can also be an Achilles' heel in corporate Web filtering policies.
David Sanders, the director of Critical Infrastructure Cyber Security, National Cyber Security Division at the Department of Homeland Security and his counterparts from the UK and Canadian governments in addition to an oil and gas industry expert will discuss "Automation Systems- An Achilles' Heel to our Critical Infrastructure.
The special teams continue to be an Achilles' heel and need more tweaking, especially on kickoff and kick returns.
Daily Forums featuring hot topics: Automation Systems - An Achilles' Heel to our Critical Infrastructure; Debunking the Myths: The Good, the Bad, and the Future of Outsourcing; and Sensors & Wireless in Homeland Security.
McGwire, 38, had four seasons ruined by injuries - an Achilles' heel and recently a Joe Namath knee.
Ovarian and breast cancer cells, like other malignant cells, may have an Achilles' heel after all.
More difficult to assess than physical damage and open to interpretation, IP represents an Achilles' heel that can be easily challenged.