Investigators for the US government have reportedly uncovered a defect in SpaceX's Falcon 9 rockets. According to The Wall Street Journal, the flaw involves cracking in the rocket's turbine blades. Those blades drive the turbopumps responsible for getting propellant into the engines.
The existence of this defect could put astronauts in danger. SpaceX has yet to send humans to space, but it plans to begin doing so by 2018.
Design changes may be necessary and they could create delays for SpaceX, which just recently returned to flight following an explosion in September 2016.
The concerns are mentioned in a draft of a report from the Government Accountability Office.
SpaceX doesn't appear concerned. It suggests the Falcon 9's design shouldn't currently be an issue, but it has already been working to fully eliminate cracking.
We have qualified our engines to be robust to turbine wheel cracks. However, we are modifying the design to avoid them altogether. This will be part of the final design iteration on Falcon 9. SpaceX has established a plan in partnership with NASA to qualify engines for [crewed] spaceflight. - John Taylor, SpaceX