A polite way of saying that an event or action may:
a) Cause an employee to experience a wealth of bad internal or external PR that could limit future promotion opportunities.
b) Cause the employee to get fired.
This definition can also be used playfully amongst colleagues suggest how unfavourable a certain option may be or the relevant risk involved.
"I'll back up that computer before I run that update on Jim's computer, accidentally breaking the CEOs computer could be *potentially* career limiting."
"I agree, stalking that cute girl from HR might possibly be career limiting."