What is the biggest lesson you have learned investing in RE?

[Tyler Solomon]

BP community - What are/is the biggest lesson you have learned investing in Real Estate? Whether you are a seasoned professional, or a brand new beginner, I believe the power in this community is learning from each other, especially each other's mistakes! 

Please share your biggest "I wish I would have known"  or  I did not realize untill..." moments over the course of your RE investment career! Tag your friends, partners, colleagues and fellow investors below - would like to compile a large thread to be utilized for the entire community here!

Newer Investors - do not be scared to drop a question below as well! Myself, or another thoughtful member from the BP community will gladly answer!