Step 1: Play Ana, get Eye Of Horus at level 10
2: Press D (to enter ADS mode)
3: Press R (to enter Eye of Horus mode)
4: Notice that your buttons have been relabeled to Q=shoot, W=cancel, E=cancel, R=shoot, D=cancel
5: Press D to cancel
6: But it doesn’t cancel anything, even though it says Cancel on it
The D to Cancel key is to cancel Aim Down Sights, not Eye of Horus. Also, the D cancel icon is ‘greyed out’ (more faded than the others), because the D key is generally inaccessible during Eye of Horus. If you press D and immediately Eye of Horus, the D cancel button will still have the normal 4s cooldown ticking down on it.