Just found something else while I was looking around. If someone is using keyboard navigation on the default phpBB registration form, they cannot access the two time zone selects (#tz_date and #timezone) nor can they access the "Anti-Spammer/Bot Test" select (#pf_antispam).
The reason, once again, is misuse of tabindex. The five inputs, and the submit button at the end of the form, all have a positive tabindex set Respectively: 1, 2, 3, 4 and 8 for the inputs, and 9 for the submit button.
Obviously, if the other inputs have tabindex set the selects should also have values set (5, 6 and 7) but since they don't they are inaccessible by keyboard. My assumption is that originally (back in the Ediacaran) the selects did have those values set, but someone cleverly removed them at some point. The result is that because focus is forced to follow the artificial order forced by tabindex, it skips from #email straight to #answer.
This is another case where the standard accessibility rule of not using tabindex values greater than 0 applies. If none of the inputs had tabindex set, the whole form would work perfectly. Focus would just follow the natural order of the markup. It would also be more maintainable and easier to customise.
There are two additional issues.
1/ #tz_select_date does not have a label.
2/ #answer does not have a label either. There is a label in the dt preceding #answer's dd parent, but it has no for attribute, so it is non-functional and from an accessibility perspective it's just confusing clutter.