The HTML being produced is invalid: Error: start tag for "LI" omitted, but its declaration does not permit this. That is, UL on its lonesome isn't permitted: it must contain LI elements.

Also worth noting that Error: Missing DOCTYPE declaration at start of document comes up at the HEAD tag; and Error: document type does not allow element "FONT" here for a FONT tag that's being used inside a PRE element.
Indeed the <ul> should be a <dl> or so for pure indents. I'll add HTML conformity checking as a todo.
Please note also that to be "identical," the second P tag should follow the /UL, not precede it. The implication as-is is that the P belongs to the UL block, when it doesn't. It may be worth using closing paragraph tags, as an aide to future XHTML compatibility, and to make paragraph enclosures wholly explicit.
This is not an Opera bug. The HTML is invalid. The blocks are overlapping, when they are not allowed to: P UL P /UL UL P /UL is not a sensible code sequence. (It should be P UL /UL P UL /UL P... giddyupgiddyup?)
I suspect this problem is pervasive, and I also suspect that the solution is almost moot; probably a one-off counting problem, or a mis-ordered "case" sort of sequence. If the /UL closing tag were to precede the P opening tag, all would be well.
Hey! That ToC thing happening at the top of this page is *really* cool!

This issue will be resolved in the course of XML formatting -- after all, XML is much more strict than any browser.