*sigh* I was hoping the macOS API would handle this correctly...

According to the log, the macOS API does return an error, and I do return an error to the rest of chromium:
https://cs.chromium.org/chromium/src/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm?sq=package:chromium&l=287

I was able to reproduce a couple of times. I'm still trying to reproduce reliably. I basically reload the page a couple of times and run garbage collection (by opening dev tools) a couple of times. After a couple of tries it breaks. I will investigate further tomorrow.

Found the problem. Fix in: https://codereview.chromium.org/2115953002

I see this problem with SensorTag which has valid characteristics too. In the screen shot, breakpoints at 99 & 100 are triggered, line 102 is never called. 

This is with, Version 54.0.2787.0 (64-bit) on macOS

Deleted: Screen Shot 2016-07-04 at 6.27.50 PM.png 87.0 KB

	Screen Shot 2016-07-04 at 6.27.50 PM.png 87.0 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7a941f9dd60b02e7b886b2774244ac5b04f1bf37

commit 7a941f9dd60b02e7b886b2774244ac5b04f1bf37
Author: ortuno <ortuno@chromium.org>
Date: Wed Jul 06 00:06:14 2016

bluetooth: Initialize start_notitications_in_progress_

This was causing StartNotifySession to hang randomly.

Also adds some logging to make sure we know what's happening.

BUG= 624745 

Review-Url: https://codereview.chromium.org/2115953002
Cr-Commit-Position: refs/heads/master@{#403831}

[modify] https://crrev.com/7a941f9dd60b02e7b886b2774244ac5b04f1bf37/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm

Your change meets the bar and is auto-approved for M53 (branch: 2785)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1b13b2d6cebb95c981c91bfe02f687ccfdf262b4

commit 1b13b2d6cebb95c981c91bfe02f687ccfdf262b4
Author: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Date: Wed Jul 13 16:04:55 2016

bluetooth: Initialize start_notitications_in_progress_

This was causing StartNotifySession to hang randomly.

Also adds some logging to make sure we know what's happening.

BUG= 624745 

Review-Url: https://codereview.chromium.org/2115953002
Cr-Commit-Position: refs/heads/master@{#403831}
(cherry picked from commit 7a941f9dd60b02e7b886b2774244ac5b04f1bf37)

Review URL: https://codereview.chromium.org/2149623002 .

Cr-Commit-Position: refs/branch-heads/2785@{#103}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/1b13b2d6cebb95c981c91bfe02f687ccfdf262b4/device/bluetooth/bluetooth_remote_gatt_characteristic_mac.mm

Tried verifying the issue by following the below steps, but could not proceed.
ortuno@: Could you please help providing other info if it could be verified from TE end.

Steps followed:
---------------
1) enable-web-bluetooth on Mac 10.11.5 using chrome 53.0.2785.21
2) Paired bluetooth to an Android device(Nexus 5)
3) Navigated to the url https://googlechrome.github.io/samples/web-bluetooth/notifications.html?service=battery_service&characteristic=battery_level
4) Clicked on start notification
5) Observed there is no device in the pop-up windows.
6) Live output was "Requesting Bluetooth Device..."

Deleted: 624745_July_19.mp4 1.8 MB

	624745_July_19.mp4 1.8 MB View Download

Thanks for trying it out! As explained in the Test Plan[1], you need a Nexus 6, 6p, 5x or 9, to be running the Peripheral app in order to test this feature.

https://docs.google.com/document/d/1jo2QyknDBsM_-nzqgdE9o0ZPR76DbHNZStXo2L5AjbM/edit#heading=h.ef8wqs9z8v4b

Based on comment#13 the verification need Android devices which aren't available with Chrome Desktop team. ortuno@ is it possible to check this at your end please.

Was not able to reproduce on 53.