The development team does a lot of testing. We have a whole test suite that is run frequently every day on numerous platforms to in order to exercise all code and make sure everything works as supposed.
Still, there are times when things aren't working the way they should. Then we appreciate getting those problems reported.
A bug is a problem
Any problem can be considered a bug. A weirdly phrased wording in the manual that prevents you from understanding something is a bug. A surprising side effect of combining multiple options can be a bug—or perhaps it should be better documented? Perhaps the option doesn't do at all what you expected it to? That's a problem and we should fix it.
Problems must be known to get fixed
This may sound easy and uncomplicated but is a fundamental truth in our and other projects. Just because it is an old project and have thousands of users doesn't mean that the development team knows about the problem you just fell over. Maybe users haven't paid enough attention to details like you, or perhaps it just never triggered for anyone else.
We rely on users experiencing problems to report them. We need to learn the problems exist so that we can fix them.
Fixing the problems
Software engineering is, to a large degree, about fixing problems. To fix a problem a developer needs to understand how to repeat it and to do that the debugging person needs to be told what set of circumstances that made the problem trigger.
A good bug report
A good report explains what happened and what you thought was going to happen. Tell us exactly what versions of the different components you used and take us step by step through what you do to get the problem.
After you submit a bug report, you can expect there to be follow-up questions or perhaps requests that you try out varies things and tasks in order for the developer to be able to narrow down the suspects and make sure your problem is being cornered in properly.
A bug report that is submitted but is abandoned by the submitter risks getting closed if the developer fails to understand it, fails to reproduce it or faces other problems when working on it. Don't abandon your report!
Report curl bugs in the curl bug tracker on github!
Testing software thoroughly and properly is a lot of work. Testing software that runs on dozens on operating systems and dozens of CPU architectures, with server implementations with their owns sets of bugs and interpretations of the specs, is even more work.
The curl project has a test suite that iterates over all existing test cases, runs the test and verifies that the outcome is the correct one and that no other problem happened, like a memory leak or something fishy in the protocol layer.
The test suite is meant to be possible to run after you have built curl yourself and there are a fair number of volunteers who also help out by running the test suite automatically a few times per day to make sure the latest commits get a run. This way, we discover the worst flaws pretty soon after they were introduced.
We don't test everything and even when we try to test things there will always be subtle details that get through and that we, sometimes years after the fact, figure out were wrong.
Due to the nature of different systems and funny use cases on the Internet, eventually some of the best testing is done by users when they run the code to perform their own use cases.
Another limiting factor with the test suite is that the test setup itself is less portable than curl and libcurl so there are in fact platforms where curl runs fine but the test suite cannot execute at all.