The news loves to pick on computer projects going wrong, the bigger the better. I won't list any here. Just google for failed computer projects or ask and AI and you will have more than enough reference points. That doesn't mean that all projects go wrong. The projects that deliver a fully functional system on budget and on time just don't catch the limelight. 

I used to work for a medium sized IT and management consultancy. We were often called to conduct an audit of projects that were in trouble. The reasons could universally be pulled from a list of the same-old causes.

• Failure to understand, document and manage requirements by both developer and customer.
• Lack of relevant management on the customer's side of the project, both when awarded and throughout the lifecycle.
• Lack of project management by the developer.
• Lack of quality management by the developer.
• Lack of risk management by the developer.
• Lack of communication and unclear communication.
• Lip service to a methodology while behind the scenes the process is 'a disorganised hack'.
• Confusing a prototype with a finished system.
• Unconstrained lust for 'shiny new technology' rather than 'stable boring safe technology'.
• Testing what is easy to test rather than concentrating on where the risks are. For example, 95% test coverage of the easy parts + 0% coverage of the high risk parts = 95% test coverage.
• Lack of documentation throughout the process.
• Lack of team structure / or / too much team structure (the first is much more likely).
• Integration with high-entropy legacy systems.
• The sales folks selling a wish list and collecting their commission, but never being held culpable for what they have sold.
• Unrealistic timescales. More developers and managers need to read "The Mythical Man Month" .

Most web development is relatively small projects. A self employed team of 1 doesn't need a team structure or a separate sales department. But don't think you are immune. These risks always apply to some degree, no matter how small the project.

As an exercise, find some of the famously failed projects for yourself and, reading between the lines, see how many of the above you can pin onto the failure. Then consider your own projects with a similar critique.