Once in a while, someone will post their theory about how the mother could be [insert previously seen character here]. The "Lucky Penny," "Something Blue," and "The Leap" statements are lines of Future Ted's narration that establish that he has not yet met the mother prior to those episodes.
- At the end of "Lucky Penny," Future Ted wraps up the story by saying that the firm he was interviewing for hired someone else, and that person had to move to Chicago three months later. He points out: "Kids, funny thing about destiny; I thought I was destined to get that job. But I was wrong. My destiny was to stay in New York. Because if I hadn't, I never would've met your mother." Aside from strongly suggesting that he meets the mother in New York, it clearly means that he had not yet, at the time of "Lucky Penny" (and some window of time afterwards, prior to when he would have moved to Chicago) met the mother.
- At the end of "Something Blue," Future Ted closes the story with this revelation of his and Robin's futures:
"And as hard as it was at the time, in the end we both got what we wanted. She did eventually go on to live in Argentina, and Morocco, Greece, Russia, even Japan for a little while. And I? Well, I met your mom."
- At the end of "The Leap," Future Ted offers up this summary of the fourth season:
"That was the year I got left at the altar. It was the year I got knocked out by a crazy bartender. The year I got fired. The year I got beat up by a goat--a girl goat, at that. And dammit, if it wasn't the best year of my life. Because if any one of those things hadn't happened, I never would've ended up in what turned out to be the best job I ever had. But more importantly...I wouldn't have met your mother. Because as you know, she was in that class. Of course, that story's only just beginning."
So anyone Ted can be construed to have "met" before the events of "The Leap" can be definitively ruled out as the mother, including Victoria, the coat check girl, the Slutty Pumpkin, Wendy the Waitress, Trudy, Stella, and most other female characters Ted has personally significantly interacted with from Seasons 1 through 4.
Notable characters who have not definitively been ruled out by this phenomenon include the Perfect Match from "Milk" (she and Ted never met in person, though they presumably know each other's names and what the other person looks like), and the Bump Girl from "No Tomorrow" (since they merely bumped into each other).
Additional (though less explicit) statements that nonetheless establish Ted as not having met the mother yet:
"No Tomorrow" - Future Ted makes a point of the fact that he learned years later that the mother attended the same St. Patrick's Day 2008 party that he did, but did not meet her there.
"The Three Days Rule" - after telling the story of how things with Holli went, Ted mentions that when he met the mother, he didn't hesitate to call her back as soon as he got her number, strongly implying that this event has not happened yet in the timeline.
"Girls vs. Suits" establishes additional facts about the mother that rule out previous encounters (and no doubts oem future ones as well). Particularly notable is the fact that he counts Cindy's description of her roommate as the first description he gets of the mother, effectively ruling out the Perfect Match from "Milk."
"Challenge Accepted" - This episode has Future Ted explicitly say that he will meet the mother the day of a Wedding and implies that to be Barney's wedding. The follow up episode, the opener of season seven, "The Best Man" confirms this as we learn that Ted will meet his wife the day of this wedding, which is in fact Barney and Robin's wedding.