How do you recover if you lose track of what you're measuring? I was making a lemon meringue pie, which called for 1/3 cup of corn starch and 1 1/3 cup of sugar, added to some quantity of water. Silly me, I put the water in the pot first, then the corn starch, and then — since I already had the 1/3 cup measuring cup out — I decided to measure the sugar as 4/3 cups. Problem is, I got distracted in the middle of measuring out the sugar, and I got to a point where I had no clue whether I had added 1 cup, or 1 1/3 cups, or possibly even 1 2/3 cups. Ooops.

Is there any way to recover from something like this? Preferably without throwing out everything and starting over?

(Obviously, the best way is to prevent yourself getting into such a situation in the first place: measure each ingredient into a separate vessel before you start combining, go ahead and dirty two measuring cups, don't get distracted, etc. But I'm sure we've all been there...)

For the record, for the pie in question I decided to quit while I'm ahead and proceeded with whatever mystery quantity of sugar was in the pot. I haven't actually tried the pie yet, it's still cooling, but it looked OK.
Going forward, as you say, it's best to use either a separate bowl/ramekin/etc. for each ingredient, or to save on dishwashing, use one larger bowl and put each ingredient in its own spot in the bowl so you can see them and figure out what's missing. 

In this specific case, though, where you've forgotten if you're at 1/3 or 4/3 cup of sugar, you could weigh the existing mixture and try to figure out by weight how much is already in there:
1. Tare an empty pot.
2. Pour mixture into empty pot and weigh it; pour it back into original pot.
3. Weigh (using tared pot) an identical amount of water (then pour it out).
4. Weigh 1/3 cup of cornstarch (and pour it back into the box).
5. Weigh 4/3 cup of sugar (and put it back).
6. Add the weights from #3, #4, and #5, and subtract the weight from #2. 
7. Weigh out sugar to match the total in #6.

This is a silly process unless you've already added something expensive (or inconvenient to replace). I've gone through exercises like this when I've added saffron to some stock, or it's midnight and I don't want to go to the grocery store for a new box of baking soda.