Considering the human digestive system as a first-in, first-out queue, it seems reasonable to view vomiting as a reversal of that queue (i.e. the first thing to come back up is the most recent thing you ate, and so on).
My question: When you eat something that causes you to throw up, can you determine the culprit by examining the evidence? Will it be the oldest meal to make the journey north, the most recent before that, or something else?
[del]Need answer fast.[/del] Never mind.
You aren’t going to like me for this. The first search term I thought of was fecal vomiting which is a rare but serious problem. That brought me to this answer which has some helpful information in it. You generally aren’t going to be able to vomit up multiple meals. Food only stays in you stomach for a relatively short time before it starts its path through the rest of the very long digestive tract. The digestive tract is partitioned off by multiple sphincters that generally only allow the food to go down and not back up. Vomit is going to consist mostly of things that you ate most recently.
As noted farther down in that link, some pathologies can cause ingesta to back up into the stomach from the duodenum or farther south and then get ralphed, but that means that you’re having some pretty serious issues (usually obstruction or ileus).
As for whether you are barfing up the culprit or some innocent bystander meal, it depends on what is causing the vomiting and how.
Some toxins induce barfing quickly by activating the CRTZ or chemoreceptor trigger zone. The CRTZ is an area of your brain that does not have the normal blood-brain barrier, allowing the neurons direct access to the circulation to see what’s going on. If those neurons contact something they interpret as a toxin, your brain sends out the “Forward, Ralph” signal.
Distention of the stomach and GI inflammation can also cause vomiting. Spoiled food may contain bacterial enterotoxins, which cause GI irritation directly and also trigger fluid shifts from the circulation to the lumen of the GI tract, causing distention, and food poisoning can set in within an hour.
OTOH, some so-called food poisoning is caused by agents such as Salmonella spp., which require hours to a couple days to do their work. These bacteria work by actually setting up shop and infecting your gut, and establishing an infection takes more time than reacting to a toxin. In this case, the food you eat right before you upchuck is unlikely to be the culprit - the spoiled food may even be out the other end already.
Sadly, our brains are programmed to fill with disgust when presented with any previously vomited foodstuff, so innocuous foods may be the target of unwarranted but very strong aversions.