What should every developer know about legal matters?
Today I had a bad surprise learning about some implications of the GPL license, mainly that I couldn't use it as freely as I thought.
Now I know.
What else should I know, and more widely, what should every developer know about legal things like that?
You can separate employees, freelancers, open source projects contributors (etc.) or give a more broad answer.
Answer
Twelve Legal Considerations for Software Development
Software is copyrighted if it is made available to the general public. It is no longer necessary to put a copyright notice on the application or in the source code. The owner of the copyright is the author(s) or company paying the author(s).
The copyright of software can be assigned by the owner of the copyright, or it can be retained by the owner and the software can be licensed to the user or users by the owner.
Libraries used in development probably have restrictions in their use and distribution. GPL does not make a library public domain, nor does the fact that the library comes with a development platform. You should read and understand the license before you distribute your application. Some libraries require royalty payments, although this has become less common in recent years.
Software patent lawsuits are crap shoots. You should not, of course, knowingly violate a software patent. However, there is a small but real chance some company will sue you for violating their patent. This may happen even if you develop your software independently, you never heard of the patent, and the patent covers a technique that is intuitively obvious and almost completely unrelated to your software. There is not a lot you can do to avoid this, given the current USPTO policies, other than buy insurance. The good news is that patent trolls generally sue large companies with lots of money.
If you use an employee or freelancer to develop software, you should make it clear, in writing, who owns the copyright to the application, including the source code. Some freelancers and contract development companies consider the source code their own property, leaving the company dependent on the original developer(s). This is legal if it's in the development agreement.
If you have an employee who develops software "off the clock," you should make it clear who owns that software, and what kind of software the employee should be able to write and distribute outside of the company.
If you are an employee or freelancer developing software, you should make it clear who will own the copyright to your application, before you begin development. Also, you should know or clarify who owns software you write on your own time. Some companies have clauses in employment agreements claiming ownership to any software written by a developer during the period of employment, whether at home or at work. Many companies have non-compete clauses in employment agreements that restrict the software an employee can produce for distribution outside the company. Sometimes these restrictions are pretty broad.
A trademark is a name or symbol, not the software itself. If you distribute software, you should (a) make sure your application name and "mark" or design of the name is not "confusingly similar" with other applications, and (b) register your trademark. Date of first use is important in resolving conflicts, so you should document when the application is first used in commerce.
When you name an application, check for registered trademarks, but also check Google. An application with first use of the name may be able to take your name and trademark after your application is successful, even if they have not registered the trademark and you have.
When you use or sign a contract or agreement, make sure both parties understand it. In an employment agreement, mentioning any potentially sensitive areas up front can prevent a lot of problems later. In a development agreement, if both parties know who owns the source code, or who is responsible for upgrades, or who is responsible for maintenance, etc., going into the development project, then there is much less likelihood of a lawsuit after the application has been completed. In a distribution agreement, make sure the distributor understands the responsibilities and term of the agreement.
Every non-trivial application has bugs (or "design considerations" :-)). Any user agreement or distribution agreement should make it clear that you are not responsible for bug-free software, and you cannot be expected to fix all bugs. Make it clear that changes, fixes, and upgrades are made at the option (or best efforts) of the developer, and make it clear who pays for fixes and upgrades.
Even after you consult a lawyer about software development and distribution agreements, you should read agreements from other software companies and see what their lawyers came up with.
I am not a lawyer, and this is not legal advice.