Everyone knows Murphy’s law - anything which can go wrong, will.

Some other laws which pop into my head from time to time:

• Moore’s law - the number of transistors in integrated circuits doubles every 2 years
• Cunningham’s law - the best way to get a correct answer on the internet is to post the wrong answer
• Pareto Principle - aka the “80:20” rule
• Hyrum’s law - all observable behaviours of a system will be depended on by somebody (XKCD)

More are collected a sites like Laws of Software Engineering and List of laws named after people.