Why didn't my lemon tart set? I am using this Smitten Kitchen recipe for lemon tarts.

I made it a few times prior (all within the last few weeks) and it worked beautifully. I loved it because it had a great balance of lemon flavour, to sweetness. The main selling point, however, was the 5 minute prep time.

In my most recent attempt, I made the pie as before - followed the directions pretty closely.

However, this time, the pie did not set. The top was caramelized nicely (I probably left it in longer than I did previous times, about 40 minutes?), but inside the pie was almost completely liquid. I have an analog oven thermometer as well, so I know my temperature was fairly accurate.

I don't do a lot of baking, so I was sure to follow the recipe pretty accurately. The only thing I changed in all three attempts, was I added an extra half lemon's worth of zest. Is there anything I may have missed that may have caused the tart to not set?

How can I ensure consistent results?

Note: On the first success I used large eggs, the second success was made with medium eggs. The failed attempt was used with the same batch of medium eggs. Not sure if that's relevant, since we used the same eggs. I wouldn't have thought it'd make a difference, other than the fact that they've been in our fridge an extra week or two.
An analogue thermometer isn't too accurate. Besides, it sounds like you measured the oven temperature, not the tart filling. So I suspect that it was the heat after all. 

A custard with both eggs and starch needs to be thoroughly cooked. The reason is that yolks contain an enzyme which liquidifies starch. It doesn't happen outright, but will happen while your tart is cooling. The only way to prevent it is to heat the mix high enough so the enzyme is deactivated. 

To quote On Food And Cooking, 


  However, the survival of yolk amylase can spell disaster in the fillings for American cream pies, which are often made in the fashion of a bouillie rather than a pastry cream, and are held for hours or days before serving, enough time for a perfect cream pie to disintegrate into a soupy mess. No matter what a recipe may say, always be sure that the egg yolks in a starch-filled pie filling are heated all the way to the boil.