Computers crashing for no apparent reason. Broken software. Code not compiling. Videoconferences. Phone conferences. Deadlines.
A PhD can be stressful. The science is challenging; we push ourselves to the limit. There are times when everything works and it's great. At other times, nothing seems to work — when I can't track down that annoying bug in my code, for example — and that is really frustrating. The key is not to give up. Be persistent. Sometimes I take a step back and remind myself of why I'm doing what I'm doing.
I've realized you must set your own standards. I'm my own boss; no one else is going to push me to get the job done. But this freedom can increase the stress level. I see many high-achievers around me who impose very high expectations on themselves. This can spill over onto others, making the environment both collaborative and competitive.
No one is immune to pressure, but it's important to maintain a sensible attitude. Why do I do this research? Because ultimately, it's rewarding. If it was easy I would get bored. As you progress, you learn how to stay sane and keep up with the pace. Well, I'm still sane and I'm still here, so I guess I'd better get back to that code.