Have you ever got a ClassNotFoundException when running tests and yet the dependency that contains the jar is clearly in your pom file? This is most commonly due to collations in transitive dependencies. If Maven cannot decide which version of a dependency to use it will not import it. It will expect you to sort the problem out first.
Handling transitive dependencies should always be done pro-actively. And there is some really good tooling for it. Being a IntelliJ/IDEA developer my self it saddens me to say that in this instance my favourite IDE is outdone by NetBeans. NetBeans uses Maven pom files as it's project descriptors which makes it very easy to open the project.
When the project is opened in NetBeans you will find the pom.xml file located under project files in the project navigator. Once opened you will find an item named "Show Dependency Graph" in the context menu. This will render a graphical representation of all the projects transitive dependencies.
If any of the dependencies has a red top left corner there is a version conflict. Resolving it is done quickest by opening the context menu for the dependency and selecting "Fix Version Conflict". Unless you have a specific preference that differs from the default suggestion in the dialogue it is usually fine to got with the default solution.
If any of the dependencies has a yellow top left corner there are more then one version of the dependency requested. Maven will always choose the latest version in the list. This can, and should, however be overridden using the exclusions tag. I tend to take control over all such dependencies buy making sure I clearly choose which one should be included. You have to do this manually by excluding the dependency from each of the dependencies where the version you do not want is included and then ensure that the dependency requiring the correct version is the only one still including it. Sometimes it's better to exclude it as a transitive dependency altogether and add it as a direct dependency.
Whenever a new dependency is added to the project I make sure to perform the same procedure to ensure that I have full control of the dependencies in my project.
I would be very pleased to find such a great tool in IDEA as well, then I wouldn't have to run two IDEs at the same time, they do require a lot of resources.