On Top of Spaghetti, All Covered in... Mold?

Dear Tricia, This isn't really a cooking question, but how long are various leftovers good for? For example, brisket vs.
potato salad, cheesecake, spaghetti, etc. Just wondering, because the mold test I'm currently using seems a bit sketchy.

Leftovers are tricky because everyone has their own rules on when to throw things out. For instance, one website I visited claims that you only have two hours from the time your food was prepared to eat it in order for it to be safe. I'm pretty sure that was authored by a hypochondriac because two hours means food served at cocktail parties are out of the question. And where's the fun in that? No one wants to pass up Vienna sausage on a toothpick. Seriously, that website (which will remain anonymous to protect their idiocy) actually said, "Some people may still feel the need to consume leftovers after a few days have elapsed...". Hey, hungry is hungry. Don't judge. So I'm not going to tell you what the health department tells you: I'm going to tell you the truth.
Unfortunately there is no throw-out-spaghetti-after-four-days rule, or brisket-only-lasts-three-days manifesto. You have to use your best judgment when guessing if the food in question is safe to eat or not. This involves using your eyes, nose, and brain. What I most commonly hear and read is to throw things out after 3 days. But I'm totally guilty of eating lots of leftovers after 3 days, especially if they're things I've made at home because then I know exactly how they were prepared. If you're just not sure about what to do (or have poor judgment to begin with), go ahead and toss 'em after 3 days. But here's some things to take into consideration.
Butter and sugar are natural preservatives, so cakes, cheesecakes, and other high-fat baked goodies are safe to eat until they mold or start to smell. I've never heard of a pound cake giving someone E. coli unless a giant E. coli monster snuck into your house in the middle of the night and rubbed itself all over said pound cake. Maybe if the baker used rotten eggs to start, and then let them sit out all night in Miami in August, and then baked the cake with those... that might get you sick. But that's highly unlikely. And check this out: most of your local bakeries let their frosting sit out at room temperature for days at a time. That's against the food code, but like I said, butter and sugar are natural preservatives. It won't make you sick, so the health department can suck it.
Meats like chicken, pork, and beef are also safe to eat until they mold or start to smell bad, as long as they've been refrigerated. If you left a steak sitting out overnight because you hypothetically "fell asleep" after too many glasses of box wine last Thursday, technically it is not safe the eat the next day, even if it doesn't smell bad. According to the health department, food can sit out for up to 4 hours in between the temperatures 40 and 140 degrees before it becomes hazardous. The health department calls this the food "danger zone," (queue the Kenny Loggins music) and meats are particularly susceptible to becoming hazardous when left out.
Seafood: trash it no later than 2 days after. If you have the guts to try and eat it after then, by all means go for it. But don't blame me when you're doing double duty in the bathroom for 8 hours.
Leaving out fruit that has already been cut up is also really sensitive. Bacteria loves fruit, so if you've made a delicious little fruit salad for yourself, make sure you refrigerate it. It's fine in the refrigerator until it molds or starts to smell bad, which usually happens after a few days anyway (Berries in particular will start to ferment after a few days, and you won't want to eat those. You could try, but I doubt they'll make it past your molars.)
Preservatives and artificial ingredients are things bacteria and mold hate. (And if I could get up on my soap box for a second, if mold and bacteria don't even like artificial ingredients, why are Americans eating so many of them? Curious.) So if you were to steam a bunch of organic kale and then put it in your refrigerator for a week, and compare that to a Dominos pizza that sits next to it for a week, the kale is going to mold and stink to high heaven. Coincidentally, this happened to my kale last week. It was nasty, and many candles were lit in the kitchen that night. But the preservatives, artificial ingredients, and stabilizers that are put into most prepared foods these days are what keep the bad stuff away. I remember one particular lecture in culinary school: if you leave shortening and butter out in your garage, only the butter will only attract flies because shortening isn't real food, its a hydrogenated, trans-fat product that mimics real food and never goes bad. And even flies don't want that. That said, your Hamburger Helper is definitely safe to eat until it starts to smell or mold.
Foods like potato salad are going to go bad much faster than the previous ones I've talked about. Anything with eggs or egg-based products (like mayonnaise) are going to go bad pretty quickly, and you never want to chance it with those. For reals, if you're not sure, then throw it out. Luckily, leftover potato salad never lasts very long to begin with. Lexi, you know I'm talking about you.

Some other things I should tell you about leaving food out:
1. Restaurant kitchens have a saying: when it doubt, throw it out. So if you're not sure if something smells bad or not, just throw it away. You're not homeless. You can make another one.
2. The time it takes to cool down food is also what makes it potentially dangerous to eat. Remember the food d-d-d-danger zone? Well if you're going to make a giant pot of soup and then put that giant pot of soup in the refrigerator, chances are the soup is going to take a really really really really long time to cool down, making it stay in the d-d-d-danger zone for a lot longer than 4 hours. So take things that are in large quantities and hot, and put them in smaller containers where its easier to cool down. You want leftovers to get cold fast, just like beer.
3. Put leftovers in airtight containers. Keeping them in the Styrofoam containers from restaurants or in plastic baggies will make them go bad much faster.

Again, I must stress that if you have poor judgment to begin with (and this is directed at those of you who ride on crotch rockets and pop wheelies while on the freeway, those of you who listen to Nickelback, and anyone who pre-ordered Sarah Palin's memoir), just throw away your leftovers after a few days. Its doubtful you're going to get sick from something you didn't get sick from the first time you ate it, but who knows... maybe listening to Nickelback while eating it could do the trick.