Not really a mistake but definitely a silly thing I used to do back in my early days:

Code:
var myBool: Boolean;
...
if myBool = true then
  myBool := false
else
  myBool := true;
First I figured out that there was no reason to type
Code:
if myBool = true then
when I could just type
Code:
if myBool then
Later I figured out simply to write
Code:
myBool := not myBool;
I comfort myself having seen many beginners write the same kind of code. It's nice to know I'm not the only one