Please feel free to edit this answer to improve it.
Criterion: Research or Academic Misconduct (e.g., Plagiarism)
Degree revocation is very rare. When it does happen, academic misconduct is usually the cause, and it usually must be severe and intentional. As henning writes:
I've been working in Academia for the past ten years. The only cases of degree revocation that I am aware of were due to severe cases of academic misconduct such as plagiarism, fraud, or large-scale cheating. The rationale is that gross academic misconduct invalidates the achievement the degree should certify.
Similarly, canadian humanist reports:
I sit on our university's Senate which is the body that would have to deliberate a degree revocation. Even a straight-forward case of plagiarism in a degree requires a long, drawn-out and surprisingly contentious decision, and it might happen once or twice a decade. This is the last step of a very, very long and drawn-out process.
So no, I wouldn't worry, outside of demonstrable research misconduct that puts the entire integrity of the degree in jeopardy.
A now-departed user provides some examples of degrees which have been revoked for major academic misconuduct, notably plagiarism:
There have been spectacular examples of falling from grace.
- Pal Schmitt resigned as President of Hungary after his doctorate was stripped by Semmelweis University on findings of plagiarism.
- Former German Defence Minister Karl-Theodor zu Guttenberg graduated...from the University of Bayreuth in 2006...A finding of plagiarism led to his degree being revoked.
The same answer also reiterates that the criteria for revocation due to academic misconduct are likely formally defined by your university:
In my university...the policy allows for a revocation in the event that the degree was awarded in error or through fraud. The policy proceeds to list the criteria and processes to be followed. Fraud is defined broadly and includes conventional definitions of academic misconduct, but other acts as well. Finally, there is a specific clause [stating that] one cannot [defend oneself] on the basis that there was no intention to commit fraud.
Practices on degree revocation also vary regionally. Sergio J Castro reports that degree revocation in the Spanish-speaking world almost never happens, even when clearly merited.
Could academic misconduct after completing the degree lead to revocation? User Stephan Kolassa addresses this:
Yes. Although you will need to do something blatantly unethical.
For instance, my alma mater (the University of Konstanz, in Germany) revoked an alumnus' Ph.D. after this alumnus blatantly falsified data, although the Ph.D. thesis as such was not tainted.
But this varies regionally. Araucaria describes a 2005 paper from a US institution, which did not find a single degree that was revoked for bad acts subsequent to graduation.
In the (2005) paper The Right of Educational Institutions to Withhold or Revoke Academic Degrees from the Stetson University College of Law Twenty-sixth Annual National Conference on Law and Higher Education, the authors give an in-depth survey of reasons universities have successfully revoked degrees. Not one of these involves the behaviour of a candidate after they have completely left the institution—whether this behaviour be academic, political or indeed criminal. [Note, Stetson is in the US].
Further, revocation due to academic misconduct is rare due to practical reasons. As reported here, it is often difficult to prove that academic misconduct took place happened years after the fact. And outside of certain high-profile individuals, few theses are scrutinized after being approved.
Prior to this wiki's creation, Academia.SE received dozens of questions from users concerned that they could be accused of academic misconduct, leading to their degree being revoked. To my knowledge, the answer was never once "yes, you committed academic misconduct and your degree is likely to be revoked." For example, concerns like data changing underneath you, the thesis not containing sufficient novelty, self-reporting possible problems with the thesis, or having a missing degree requirement were all dismissed.