Add note about Netskope outliers

.spelling

@@ -255,6 +255,7 @@ npm
 NSIS
 NumPy
 num
+Netskope
 oauth
 OKR
 onboarding
@@ -461,6 +462,7 @@ sql
 Unioned
 1
 2
+4xx
 unioned
  - src/datasets/bigquery/firefox_android_clients/reference.md
 first_session

src/concepts/analysis_gotchas.md

@@ -30,6 +30,7 @@ When you start to evaluate trends, be aware of events from the past that may inv
 
 - **Apr 18, 2024** - Google began rolling out a new SERP experience on Firefox Android, which impacted standard search metrics. See [the spreadsheet](https://docs.google.com/spreadsheets/d/16Cyx_KBieRdQkSBKolivqpBaK2H-VceN9LEZcL0snHg/edit#gid=0) for more details.
 - **Jan 15 - May 1, 2024** - Legacy Telemetry pings containing os information from Arch Linux clients without the `lsb-release` package were [dropped](https://bugzilla.mozilla.org/show_bug.cgi?id=1875874).
+- **Dec 8, 2023** - (ongoing) Firefox iOS Clients coming through the French ISP Netskope report an increased number of 4xx HTTP errors on submission ([bug 1870670](https://bugzilla.mozilla.org/show_bug.cgi?id=1870670)).
 - **Dec 7, 2023** - Contextual Services data for Firefox Desktop versions 116 and up [now supplied by Glean](http://mozilla-hub.atlassian.net/browse/DSRE-1489).
 - **Nov 20, 2023** - Changeover day for Onboarding data sent via Messaging System from PingCentre to Glean. Views and datasets downstream of `messaging_system.onboarding` [began being fueled by Glean-sent data instead of PingCentre-sent data](https://github.com/mozilla/bigquery-etl/pull/4457).
 - **Jul 16, 2023 - Jul 24, 2023** - During the migration from release to ESR of Firefox users on obsolete versions of MacOS and Windows, Firefox sent deletion request pings for clients in the migration, which also reset the `client_id`. [See the summary of the incident here.](https://docs.google.com/document/d/1vdn9OFSoKPD5wt14dmTwyh0kGs-96fWx26ESui95jo0/edit). Approximately 2 million clients were affected by this bug; as a result of this, around 1.3 million clients were double counted due to both the old `client_id` and reset `client_id` being active on the same day.