http/tests/media/reload-after-dialog.html and http/tests/media/video-cancel-load.html are "crashing" on the Debug bots, which presumably means they're asserting. See the URL for one such case.
I've added these tests to the Skipped file.
Now that we're skipping these two tests, http/tests/media/remove-while-loading.html has started asserting. That test comes right after http/tests/media/reload-after-dialog.html, which makes me think it is actually the previous test (http/tests/media/pdf-served-as-pdf.html) that is causing the assertion.
I've replaced http/tests/media/reload-after-dialog.html with http/tests/media/pdf-served-as-pdf.html in the Skipped file. We'll see what happens!
Unfortunately I can't reproduce the failures locally as Apache seems to hang on my machine.
See also bug 47093.