To your disappointment you discover that the post containing the original image is nowhere to be found on the page you landed on. Shockingly, the blog author has moved on to create literally dozens of new posts in the 33 weeks since the image was first pinned. Now you've got to go out of your way to do additional research to try to find the original post. This truly puts the word UGH into UGHLY!
That said, I thought it was worth taking a moment to describe how to properly pin images from a blog so that you don't completely annoy the world with hap hazard pinning habits. Firstly, 99% of blog authors create titles for each post. Post titles act as a direct link to the actual post webpage where you can (and should) pin from. When you find an image you want to pin, make sure the webpage you are visiting contains only the address of the post with the image you want to pin. How do you know? I'll tell ya.
If you are visiting a regularly updated blog, generally you will see many posts when you scroll down. If you try to pin from the blog home page which looks something like this: www.someblog.blogplatform.com you're going to be presented with a plethora of posts. This page is basically a list containing the latest posts first so of course, if you pin something linked to this page today, 12 weeks from now the page that pops up when we click your pinned image is not going to look the same as it did the day you pinned because newer posts have been published. Get it?
On the other hand, if the web address in your browser window says something like this: www.someblog.blogplatform.com/2012/12/actual-article-to-pin-from.html then it's a pretty safe bet you are visiting the actual post. From here you will also be able to see any comments so that's another clue you are in the right place. If you find that you are on the home page, all you have to do is find and click the post title to be taken to the actual post page. This might sound like extra work now but keep in mind that the someone looking for that great image later, that goes to that awesome, must make tutorial could be you and how frustrated will you be if you can't find it?
Sidebar images are a whole different story. If you pin a sidebar image from a blog home page then we don't have a problem because as we scroll down the page we'll be able to see that image and can click on it (if clickable) to get more info. End of story.
I realize that there are many who pin because they like the pretty pictures but there are a lot of us who actually visit the sources later and a pin from a blog home page is completely useless to anyone trying to find that image weeks or months later. It's frustrating so just be aware of what you are doing. Happy pinning! :)