Just as well..
.. they weren't racing up to save Atlantis!
NASA scrubbed the planned launch of space shuttle Endeavour early on Saturday morning due to "a leak associated with the gaseous hydrogen venting system outside the external fuel tank". The agency explained: "The system is used to carry excess hydrogen safely away from the launch pad at NASA's Kennedy Space Center in Florida. …
Hydrogen can be a bitch to work with- even airtight seals won't hold necessarily hold it (see the rate of deflation for an H2 balloon over an air one at similar pressure) and you need non-H2 reactive sealing materials.
However, you'd have thought that NASA could have sorted these problems in 20 / 30 years. Then again the Shuttle never was a serious space vehicle.