I Feel Violated!
The GPL is several pages of legalese. I've read it several times, and still have to look up specific things, and I probably have a better knowledge of copyright law than the average developer. One of the major selling points of Free Software is that there's no cost in tracking licenses. Any piece of Free Software comes with the FSF's freedoms 0 and 2, meaning that you can use it and copy it as much as you want. This arrangement dramatically simplifies auditing, and is a huge win for big companies.
Because the GPL is a Free Software license, the same principle applies here, right? Yes, as long as you distribute with the source. There's a simple way of violating version 2 of the GPL, which I would imagine at least half of the readers of this article are guilty of doing. Grab the source code for your favorite GPLv2 project, compile it, and give a friend the binary. Oopsyou've just committed copyright infringement. The GPLv2 requires you to provide either the source or an offer in writing (good for three years) to provide it (see clause 3).
It's generally considered helpful to compile software for your friends who aren't particularly computer-literate. They have no interest in the source, but the GPL requires you to give it to them (they'll thank you for it when they download the combined package over a pay-per-megabyte mobile connection), or give them a written offer for the source. If you choose the second option, you need to keep a copy of the source around for three yearsand don't forget that it needs to be the exact version used to produce the binary.
This is only an individual concern, right? Well, not really. Imagine this conversation taking place between two employees of different companies:
"I can't open that file you sent me."
"Oh, sorry. I'll send you the program I used to make itit's free."
Sound reasonable? Variations of this same conversation are fairly common when the software in question is proprietary and it's illegal to provide copies of it, so for Free Software this conversation is likely to be even more common.
This helpful employee has now opened his or her company to legal liability. Note that in this example neither person modifies the source code at all, they both just use and share it. These are two of the conditions defined by the FSF as being vital, yet by exercising these freedoms, they have violated the license.
Of course, no reasonable developer would sue for this kind of GPL violation. Unfortunately, there's no trivial way of testing whether the developers of a particular program are reasonable.