The importance of March 5, 2005, which is the date of Ellie Oswald; Clara’s mother’s death, was originally covered here. It’s known that it’s the day that The Doctor first meets Rose Tyler. Well, going back and watching Rose has possibly lead to another clue regarding Clara’s mother (apologize for video quality):
Rose states that there was a body found in the building from the explosion. The body found is assumed to belong to Wilson, who Rose was looking for and The Doctor said was dead. However, its never stated by Rose or anyone else who the body belongs to, leaving the door open for the body that was found, to be Ellie Oswald’s. Meaning that The Doctor is responsible for the death of Clara’s mother.
Where this leads, if anywhere, I’m still unsure of. One thought I’ve had is that piece of information, while random, could be determined by an enemy, especially one such as the Great Intelligence. Something used to try and turn Clara against The Doctor? Over thinking it? Who knows!
UPDATE: We talking a little about this during the latest episode of Police Box Paradox