Name your Critical lesson learned?

[mis_chiff]

Share your first, most critical lesson learned.

Whether it was a financial loss, a TM issue,
or a hacker intrusion, or a serious development issue,
example: where you get content, or fonts,
or publishing items found elsewhere, etc

Mine:
When I first started out, I had no idea what TM meant
(I hadn't even heard of it...yes I was a noob!)
so I found MichaelJFox.ca was available to reg and
thinking I had a "gem" I registered it!
Then I started reading through
some threads here on Namepros and realised...
I had just registered a TM'd name!
So needless to say I was a little nervous thinking
the domain TM police were going to knock on my door (j/k)
but I cancelled the transaction right away,
and got my money back.
I figured it wasn't worth the risk, even though I see it being
done...it wasn't for me.

So what was your first critical mistake, maybe we can all learn
something from it, as I did