Turns out that Cave Johnson is willing to admit that there is a reproducible human error problem with the portal device. It’s all due to the way humans tend to land badly, especially from great heights, and the resulting damage that can occur to the portal generator.
Needless to say, in order to protect the device, the boffins have come up with a safety system. Want to help? Just send cash to support further research. Remember; it’s for science, you monster.