From https://discourse.phabricator-community.org/t/backspace-in-empty-assigned-subscriber-box-causes-crash/1520
Reproduces for me here.
In New Task form, click in (empty) "Assigned To" tokenizer and hit backspace - tab freezes.
Also happens via the "Change Subscribers" action in the action panel.
I'm in Chrome/Windows, other user reports Firefox.