On 25 June 2010, the Eleventh Doctor began his travels with Amy Pond. When they later returned, Amy attempted to seduce him and they picked up Rory Williams at his stag party. (TV: The Eleventh Hour, Flesh and Stone, The Vampires of Venice) Amy and Rory got married the day later, the day the universe fell apart. (TV: The Big Bang)
On Monday, 25 June 2007, at 10:01 AM, Donna Noble made a seemingly tiny decision by turning left on Little Sutton Street, heading for the Chiswick High Road, ignoring her mother's pleas to work for Jival Chowdry and turn right, towards Griffin's Parade. In the long run, this decision saved the Earth several times over. Her job at H.C. Clements resulted in her meeting the Tenth Doctor and saving his life beneath the River Thames.
A Time Beetle on Donna's back created a parallel world, where Donna turned right instead. This meant her never joining H.C. Clements and thus never meeting Lance Bennett and becoming engaged to him. Instead, she took a job as Jival Chowdry's personal assistant. Six months later at Christmas, the Tenth Doctor died because of this and the whole world turned to chaos. (TV: Turn Left)