The only way that you can damage your phone by jailbreaking is by doing one of the following:
- Not being able to follow the very basic instructions when using Evasi0n or any other jailbreaking software
- Forcing applications/tweaks on your device despite the warnings that it's incompatible with your device with the software/hardware check
- Not reading the descriptions to apps and tweaks and see if they're compatible if number 2 is not applicable
- Snooping around system files when you have no clue what you're doing
- Disconnecting your device during a restore process or during a jailbreaking process
- Don't overthink
- Do not maintain and groom your jailbroken device
It's really REALLY hard to mess up a jailbroken iDevice. In most cases, you'll get hiccups, and those are easily solved by using Safe Mode or simply restoring your iOS. Then there are the bigger problems such as users not following directions and heeding to warnings that tells you to do this rather than that. If you're unsure about something
ask the question, there aren't any stupid questions, just stupid answers, meaning you have nothing to lose. Better to get a plain one word answer, than to completely and utterly screw up your device. If you want to mess up a jailbroken iPhone, with a little common sense and a bit of reading, it's as easy as explaining the theory of relativity to a 2 year old, which is not at all.
One example would be users trying to force tweaks and apps to run on their devices by changing the profile of the device. Sure you'll be running that app or tweak, but it's not made for your device and was never compatible with your device. Say the app is for iPads only, and you forced it on an iPhone. It might somehow work, but after a respring or reboot, you'll have yourself a nice, shiny, glossy, expensive brick.
