Can you share your favorite quote or rule related to IT?

Not restricted to IT, but anyone who has tried estimating work can relate to this:

Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law

Hofstadter's Law

Coders are special. "We are expected to know how to do things we've never done before and estimate how long they will take."

Users lie. They may not be lying about an issue, how they ran into the issue, or how the bug is affecting them but assume there's at least one lie in every bug report. It may be something completely unimportant. They have no reason to lie, but they will.

• A past manager of mine.

The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.

-Tom Cargill

There’s even a Wikipedia page about the 90-90 rule.

Individuals and interactions over processes and tools

The first line of the Agile Manifesto is the one that gets ignored the most.

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, be definition, not smart enough to debug it." - Brian Kernighan

"Weeks of coding can save you hours of planning." - Unknown

There's nothing more permanent than a temporary solution.

-- Howard Cauvel

Make it work, make it right, make it fast.
-- Kent Beck

My other favorite is this:

if you ever code something that "feels like a hack but it works," just remember that a CPU is literally a rock that we tricked into thinking
[...]
not to oversimplify: first you have to flatten the rock and put lightning inside it
-- @daisyowl

Always code as if the guy who ends up maintaining your code will be a
violent psychopath who knows where you live.

John F. Woods