My least successful project was:

social network for student exchange. I did it for the student organization that after launch decided that they really don't need that and shut it down.

Idea was for students to subscribe to internships of their interest (for example 'developer internship') and then they get notifications when someone from the network publish internship within subscribed interests.

I'm still a student so maybe my experience isn't really interesting, but I had this project for a semester where I had to create a library management program, It was a mess because our teacher kept changing some details about the project and also because my lack of knowledge about some key functions that had to be there, so in the end it kinda came together and did mostly work when we had to show it for the final evaluation.

I did however learn a lot while working on it and the feedback I got from our teacher was useful.

@Diego35HDs personally, your project sounds interesting and I also tought about library mangement app as I am librarian!

About changing specs of the project, well that kinda happens when client doesn't know what he wants, happens a lot in this line of work, so i guess it was useful experience! =)

@devJs Indeed! Now that I think about it maybe it was part of the challenge so we're ready for the real deal

And about the project, it was made in Java and used MySQL, you could register books, clients, authors, etc. The most important parts were exception handling, validating stuff like id numbers, ISBN, String lenght and int values, and making it user friendly with comboboxes that filled with content taken from the database automatically, an easy to understand GUI and of course the user registration and login

That about sums it up, sorry if parts of my comment are hard to understand or seem poorly written, english isn't my first language, but I'm trying my best hehe

@Diego35HDs your English is pretty good actually, don't worry.