1. Microsoft or McAfee or Amazon or Netflix asking you to call them about a $400 refund or cancellation.
Microsoft, McAfee, Amazon and Netflix will not ever email to warn you about a $400 refund. And if they do, it is a scam. They definitely wouldn't leave a phone number in the email for you to call.
Don't call that number. The way this scam works is that scammers get you to call a number because they have scared you into thinking you have been charged $400 by mistake and that they are going to help you get a refund. But instead they are going to have you share access to your computer and trick you into thinking that you have accidently been refunded $2,000 and that now you now owe THEM $1,600! Then they will plead with you to send them the $1,600 in cash, or by buying $500 Walmart gift cards or by visiting a BitCoin cash machine at 7-11. I know it seems like you would have to be crazy to think someone would just send $1,600 to someone they don't know, but their trick is to make you think that you are returning their money because you are a good person.

2. Strangers on Facebook Marketplace or Craigslist sending you a check for your item plus some extra money for shipping and fees.
Real people do not send real checks to strangers, and if they do, it is a scam.
The way this scam works is that scammers send you a bad, fake check that is for $180 over the price of your couch or car or whatever you are selling. They say, "This is for the car, but I've included extra money, so send me the extra money back".
You deposit the check, take out their money, send them cash and your item, and they four days later the bank tells you the check was bad and the money doesn't exist.

3. Fake friend on Facebook, Tinder, Bumble etc. falls in love with you without ever meeting you.
People do meet and fall in love over Tinder and Bumble, but they always see each others' faces in video or in person.
The way this scam works is that the fake boyfriend or girlfriend works in a submarine or on an oil rig or in outer space so that they can NEVER meet in person. They will have good, sexy photos. The photos are stolen from a model or actor.
Often the scammer will ask for you to download a new chat app, like Whatsapp or Telegram or ChatKing that has free chatting and no anti-fraud measures. This is a sure-giveaway of a scammer.
The fake lover will exchange messages with you for a long time, perhaps even weeks, but then they will have a fake financial crisis "My son needs surgery" or "My mother is in jail" and will ask for $2,000 send via Western Union to help them. This is a scam.

4. Fake plea from your nephew Brad who is in big trouble.
If someone can trick you into thinking your grandson or nephew needs a bundle of money immediately, you might send money to a scammer.
Facebook and other social media platforms can reveal the names of your kids and other family members, which is fine, but a scammer can get that information.
This scam starts with someone calling pretending to be a jailer, judge, tow yard, police detective, DEA agent or French ambassador who has your grandson held in detention. The scam continues with the description of a time-sensitive crisis, and an inability for the relative to speak to you directly. The scammers insist you can only help him by sending a code from a gift card or filling out credit card information online.
The best way to avoid this scam is to hang up and call your nephew directly. Then call his parents. Most likely, your nephew is safe at home, playing Xbox.

5. Fake call from the IRS that you are in big trouble.
The IRS doesn't call anyone out of the blue.
Scammers pose as IRS agents, call your phone and demand immediate payment of $1,500 or threaten to have the police come immediately and throw you in jail for tax evasion.
From the IRS website:
"Note that the IRS does not: Call to demand immediate payment using a specific payment method such as a prepaid debit card, gift card or wire transfer. Generally, the IRS will first mail a bill to any taxpayer who owes taxes. The IRS does not demand that you pay taxes without the opportunity to question or appeal the amount they say you owe. You should also be advised of your rights as a taxpayer. The IRS does not threaten to bring in local police, immigration officers or other law-enforcement to have you arrested for not paying. The IRS also cannot revoke your driver’s license, business licenses, or immigration status. Threats like these are common tactics scam artists use to trick victims into buying into their schemes."
In the last few years, fake IRS scammers try to scare people into buying Bitcoin at ATM machines and transferring it to the scammers so it cannot be recovered.

6. Your boss asking for gift card code
Your boss does not need a gift card. There are no gift card emergencies.
This scam works with a fake email that appears to be coming from your boss, VP of marketing, or the CEO, etc. The fake boss asks you to use the company credit card or petty cash to get a couple of $250 Walmart gift cards. Then he will ask to email the codes on the card back to his email address. The scam is that the email isn't actually coming from your boss, it is coming from a scammer, and all he needs is that code to redeem the card in another state or country.
The best way to avoid this scam is to talk about it with your boss before you ever get such an email. Don't let a fake sense of emergency or secrecy let a scammer trick you. Verify the information with your boss by contacting him through another method.

I used your grandparents in the title, not only because they are somewhat more likely to believe in a voice on the phone, but because they probably have fewer peers to consult and more money to lose.