I was thinking about the episodes from the Poll I just made and I had a thought- Wouldn't it have been a nice callback if Martha had shown up in Hostage to surprise Clark for his birthday? Of course, I know she was there for all the Red Queen/Checkmate stuff, but it would have seemed less random, in my opinion. Clark's birthday was celebrated in Calling and in Oracle.
Calling
Oracle
I guess it would have been even worse if Lois had broken up with Clark on his birthday though...
The screen caps are courtesy of
Smallville Dedication.