Planning Ahead

Some of your seemingly unrelated, random choices may cause you to be unable to access the necessary quest in order to win the affection of the person you want to marry.  I'll be compiling a list in this post as I discover what some of those may be.

Sadri / Viola
The good news is that there is only one favor quest which actually makes another marriage candidate impossible to marry.  This is the favor which Revyn Sadri will ask you to do for him if you ask if anything in his shop is stolen.  He will tell you that he bought a ring by mistake which he believes actually belongs to Viola Giordano and requests that you sneak the ring back into her house so no one will know what really happened, or that he was involved in any way.  If you choose to hand the ring over to Viola directly she will become available for marriage, but Revyn will be out of the question.  If you choose to secretly plant the ring as Revyn requested then he will be available for marriage (and also for speech training), but there won't be anything else you can do to raise Viola's disposition toward you high enough to make her available.  Luckilly since they're not the same sex (not to mention race or type of person, etc.) it's unlikely you'll be trying to decide between those two.  Still, it might be easy to make the mistake of helping Sadri without realising that it's excluding Viola as a marriage prospect. 

Ysolda
It's very easy to accidentally rule her out.  According to this site:  http://www.uesp.net/wiki/Skyrim:Ysolda if you do the favor for Carlotta at the vegetable stand in Whiterun (telling the bard Mikael to stop bothering her) before you've obtained Ysolda's favor quest to retrieve a Mammoth tusk, she may not give you the option to do this for her, making it impossible to win her affection.  In my experience this certainly seems to be confirmed, so if you have any interest in Ysolda, be sure to speak with her first!  --  Sooooo, after many many in-game hours have passed (from level 3 to level 60), and countless other quests done, Ysolda finally gave me the dialogue for her mammoth tusk quest.  I'm not sure why she suddenly did, although the most recent thing that might possibly be related is that I finally went ahead and completed the Bleak Falls Barrow/Golden Claw quests which I'd been putting off because I didn't want to deal with dragons.  Possibly returning from that had some effect?  I speculate this because it was right after this that I saw her on the steps to Dragon'sreach for the first time, which is also where I obtained the quest from her with my other character.  So... it's possibly related. 

Sorex Vinius
It's all too easy to loose the opportunity to win Sorex's favor, especially if you head straight to the shops after entering Solitude.  Sorex is the son of the owner of the Winking Skeever and can either be found in the inn or browsing the stands in the outdoor market.  He will ask you to do him a favor by delivering some rum to Falk Firebeard.  Unfortunately, according to this site (once again): http://www.uesp.net/wiki/Skyrim:Sorex_Vinius the speech option for his favor will not show up if you agree to do a favor for Angeline Morrard in the Alchemy shop first.  I can confirm this issue as well.

Brelyna Maryon
If you join the Mage's Collge in Winterhold, Brelyna will ask if she can experiment on you.   Agreeing will gain her friendship.   The trouble is that, apparently, if you have any kind of Spell Absorbtion effect at the time (according to this site: http://www.uesp.net/wiki/Skyrim:Brelyna%27s_Practice), you can get stuck in the "wait for Brelyna's spell to wear off" stage permanently and can never become her friend (or spouce).  I have had an issue with this myself, although I don't believe I had a spell absorbtion effect.  Instead I speculate that a Resist Magic effect might interfere with this as well.  Thus I advise doing this favor for her before you take any Resist Magic Perks under Alteration, and before becoming an Agent of Mara.  I will have to do further experiments to confirm this.