An example of where patents can cause a great deal of harm is the .gif format files. Gif files were widely used for years. Unisys discovered that it had a patent that covers the .gif format [16]. In particular, Unisys writes:
Unisys has frequently been asked whether a Unisys license is required in order to use LZW software obtained by downloading from the Internet or from other sources. The answer is simple. In all cases, a written license agreement or statement signed by an authorized Unisys representative is required from Unisys for all use, sale or distribution of any software (including so-called "freeware") and/or hardware providing LZW conversion capability (for example, downloaded software).The .gif format uses LZW compression. You can't use this technique without a written license from Unisys. Even if you discovered the same technique on your own you could still be sued and your program would have to be withdrawn from sale.