Major preface here, I support the antiwork movement and have for a long time. I believe employees on all levels are incredibly mistreated and a major movement could result in life altering changes. Also, I'm focusing on the US since that's my expertise…
For a while I have seen countless posts of an employer doing generally shitty things that are within legality, but still incredibly shitty. All I will see in the comment section is “that's illegal, you should take them to court” or “DOL would love to see this!” the reality is, the DOL wouldn't care because laws allow a lot of shitty behavior.
Didn't work mandatory overtime when you only got 12 hours notice? What does your handbook say? If it says you need to be able to cover unexpected shift openings, you can be terminated. Full stop
Have a medical condition and can't attend work for 6 hours a week? Are you able to complete that work without it being an undue burden on the company? No? Terminated. (it's shitty, but legal)
Boss went on a power trip and fired you for what seems like no reason? You're probably in an at-will state. Legal.
Time rounding on your clock in/out at work? Legal as long as it's seen as fair. Even if it's not fair you report it, company gets slapped on the wrist and a minor fine, then moves on. (California is an exception there's more limits there)
If you don't actually know the laws, don't say it's illegal. It probably isn't. Shitty yes, illegal no.