Andriy Slynchuk has described eleven things that Internet users may do that are illegal. The five most serious violations, in my opinion, are:

1. Sharing copyrighted material without permission: This is a serious offense because it violates the rights of the copyright owner and can result in legal action.
2. Accessing protected computer systems without authorization: This is a serious offense because it can result in the theft of sensitive information or damage to computer systems, which can have significant consequences for both individuals and organizations.
3. Engaging in cyberbullying or online harassment: This is a serious offense because it can cause emotional distress and harm to the victim, and can also lead to serious legal consequences.
4. Distributing child pornography: This is a serious offense because it involves the exploitation of children and is illegal in most countries.
5. Creating or distributing malware: This is a serious offense because it can cause significant damage to computer systems and networks, and can also result in financial losses for individuals and organizations.

These offenses are serious because they can cause harm to individuals and organizations and can also result in legal consequences for the perpetrator. It is important for Internet users to be aware of the laws and regulations related to their online activities in order to avoid committing these offenses.