There have been many, many, many over the years.....one that pops out right away is this:
Target exclusive #4534 train set...
I found this on a shelf years ago...it was not in their system. (had no idea what it retailed for at the time either) Brought it up to the cashier, she radioed to a manager who looked up the bar code further in their system, and said over the radio...$40, 75% off. Both me and the cashier thought she meant 75% off of the $40...I didn't understand how it was so cheap but got it for $10. Later I realized it retailed for $160 and the $40 was the clearance price, not the price to take 75% off of...oops. (I was probably like 13 years old)
When Lego clearanced out their 9v train track....I bought about a dozen of the 4515 straight rails for my collection....got them for $4.23 a piece. I ended up selling a few left over that I never opened on Amazon for about $75 a piece last year.
Lots of garage sales and random other finds as well....about a month ago a friend of mine found a large box of random sets for $40...he called me up straight from the garage sale and said there were "train parts" in it. He then said "she wants $40 for it and she won't budge...oh I see a part that says "Santa Fe"."
At that point I was like "BUY IT!!!!!!!!!!!!!!!!!!!!!!!!"
I told him I would pay him for the box but he ended up just giving it to my since it was my birthday the week before. I still haven't really looked through much of it since I've been so busy, but there are instructions for the Sante Fe engine (and the engine is about 95% built), the large train shed, another train set, and random other sets and pieces. This should definitely turn out to be one of my top finds once I get some time to look through it.