So far, we've hit this 6 times, all 6 of them on opt linux64-qr or linux64-shippable-qr builds. Bug 1574031 might be similar (adding to see also). That one is also opt-linux-only so far (both -qr and not-QR, in that case).
Bug 1574306 Comment 2 Edit History
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So far, we've hit this 6 times, all 6 of them on opt linux64-qr or linux64-shippable-qr builds: https://treeherder.mozilla.org/intermittent-failures.html#/bugdetails?startday=2019-08-13&endday=2019-08-16&tree=all&bug=1574306 Bug 1574031 might be similar (adding to see also). That one is also opt-linux-only so far (both -qr and not-QR, in that case).