I've seen it mentioned on this site many times, but I dont think it can be stressed enough. If not Visa related enough please delete.
ALWAYS check your entry stamp. I just came from CW to get a 30 day extension on my 60 day entry stamp. The original date was for April 27th. I was given a stamp for May 28th. I looked at it but it did not register that it was 31 days, not 30. Luckily they noticed it and called me to tell me I needed to go back before I had left the building. She gave me a new stamp and I started to leave the office before double checking. They had made an error again and gave me a stamp for April 27th, the exact date as the original stamp. After going back they finally gave me the correct stamp for May 27th. I would suggest doing the math ahead of time and knowing exactly what it should be so you can check and have it corrected if need be before even leaving the counter. They process hundreds of people per day, with different dates so it is easy for them to make a mistake.
Other than that everything went smooth and quickly. I had all my documents in order thanks to the advice I got from reading replies to other posts on this site. Thank you to all the admins that take their time to answer all the questions.