Sailor Moon featured time travel and the rewriting of what happened on occasion, so there were a number of times when an alternate timeline was seen. After the introduction of Chibiusa, the "past" no longer appeared to be changeable, as the death of either parent could not rewrite the second half of Sailor Moon R.
Anime: Season 1
At the end of the first season of the Sailor Moon anime, Usagi Tsukino wished on the Silver Crystal to be a normal girl. Though there was never any official explanation of this wish, while trying to resolve the overall timeline of the series, many fans have speculated that the wish ended that timeline and the R season was a timeline as if the first season had never happened; the only difference was that the characters had remnants of that timeline buried in their memories, and Luna and Artemis still retained all memories of what happened.
Sailor Moon Sailor Stars
During the Stars season of the anime, an alternate timeline was not actually shown, but in theory existed during the time Mamoru Chiba was dead. Chibiusa was returned to the future just before that occurred, so the Senshi couldn't realize that she had ceased to exist.
The Tomoe Family
Hotaru Tomoe was killed and reborn, but Professor Tomoe's perception and acceptance of her baby self may be a product of an alternate timeline placed into his memories by Sailor Moon. This is uncertain, however, due to the fact that it may very well be amnesia caused due to the stress of the events of the Infinty storyline, but it is still worth noting.
In the future Crystal Tokyo, when Chibiusa returned to the 30th Century the Inner Senshi were shown wearing their first sailor fuku instead of the ones they wore in later seasons. Also, the scepter that Neo-Queen Serenity carried changed according to what was being used by Sailor Moon in the 20th Century timeline. It is possible these were plotholes, but could also have indicated that the future was still changing.
Pretty Guardian Sailor Moon
At the end of the live-action series, Usagi again wished on the Silver Crystal, but it briefly rebooted the world to an unspecified timeline where Usagi and Mamoru never existed, but the other Senshi retained some character development attained when they knew her. The girls never befriended each other but Naru Osaka and Ami Mizuno became friends with each other in the absence of Usagi.
Usagi and Mamoru were able rejoin this timeline, restoring everyone's memories of what happened during the series.