In Fuchsia we typically send up a blank image which will carry test results, have Fuchsia running in QEMU modify that image, and send it back. This blank image generally doesn't change, so it's very heavily cached by isolate on the bot, and swarming bot will just symlink the file in instead of doing a full copy for performance reasons. The cache is also not kept consistent. As a result, we see build successes like the following:

https://luci-milo-dev.appspot.com/p/fuchsia/builds/b8954575410346597648

None of the tests listed in the steps toward the bottom  exist for builds generated by this builder.

For now we're working around this by copying the file explicitly before we execute QEMU.