On Java: it had been a common sightseeing in the IANA tz mailing list that some complained about TZUpdater [1] not working. It's amusing if not annoying, and the root cause traces back to the intricacy of time zones themselves.

Apparently some countries at some points had (and sometimes, like Ireland [2], have) a "winter time" as opposed to a "summer time". In the other words their standard time takes place in summer and the "winter time" has a negative offset to the standard time. Technically this is not different from a "summer time" in other countries, but C `struct tm` has a `tm_isdst` flag that ideally should be true in winter for those countries.

tzdata made transition to support negative DST offsets in early 2018, but this caused unwanted software problems so the choise was made to support the "rearguard" format that doesn't have negative DST offsets. This was an one-off incident for most cases, but TZUpdater didn't support the more recent (and more correct) "vanguard" format. And yet users tend to feed the vanguard format to TZUpdater and thought tzdata was at fault. This annoyance continued until TZUpdater finally supported the rearguard format in late 2019 (I think).

[1] https://www.oracle.com/java/technologies/javase/tzupdater-re...

[2] https://en.wikipedia.org/wiki/Time_in_the_Republic_of_Irelan...