As originally reported on MDL-70093:

When a cell is very close to a page long and displayed on a page on which a new page header is generated (such as any page following a very long cell), it results in rowheight problems and visible overlap yet again. I expect that some combination of the header and page break space (see screen shot where I've highlighted this) is at play here.

The interesting thing is that this 'page break' space/margin (or whatever it's called) is only applied the first time the content crosses a page boundary. If the cell spans many many pages, you'll notice only the end of the first page gets this margin.

I've attached a screenshot which demonstrates this problem. It's essentially the same with or without the patch (the erroneous heading is removed with the patch, however) and, I suspect is another issue entirely. Thought it worth reporting anyway.
To replicate this in a new forum, just follow the steps below:
1. Create a new forum
2. Create a post with exactly this content:

3. Create a reply to the above post, using the exact same content.
4. Export the forum as a pdf and note the overlap.