David Goodman wrote:
Which of the preference settings are likely to cause
this problem?
The preferences that can -combined- make you unique[1] are:
math preference, date format, auto-number headings, user language, image
thumbnail size.
Interestingly, the stub threshold supposedly does not affect it [2].
Additionally, being or not in the secure server acts as another preference.
1-Taken from User::getPageRenderingHash()
2-That is what a comment there says, but take it with a grain of salt.