A rollback is an operation which restores a database to a prior state. In Growtopia, this affects worlds, player inventories, gem counts, and stats, usually in order to revert the effects of a game-breaking issue which cannot be resolved in another manner. Consequentially, any work which players put in during the timeframe which was rolled back is lost.
In compensation for lost work and time, rollbacks are often accompanied with an Apology Weekend, a weekend-long event in which in-app purchase values are increased and players have a chance of getting lucky drops from breaking blocks. Any gem purchases made within the timeframe which gets rolled back are given back to the player at double the value.
The first rollbacks occurred on February 23rd of 2013, and resulted in 24 hours of data to be reverted twice. The second rollback occurred only a few hours after the first, and sent players back to the same place they were rolled back to the first time.
The occurrence happened due to an oversight where store-bought items which had a rarity of 999 were able to be recycled at a significantly higher rate than the purchase price. Players caught onto this and took advantage of it, to effectively gain an unlimited amount of gems. This completely ruined the game's economy at the time. A rollback was issued to restore the game to a state before the exploit was first discovered, and the exploit itself was fixed by adjusting the store items to not recycle for any gems, though they were changed much later to recycle at 10% of the original price. The shop items were also set to not display a rarity.
The Apology Weekend which followed this ordeal granted players to splice the Rollback Plaque in the duration of the event. It was made by splicing a Cave Background Seed with a Door Seed. After the weekend, the recipe would make the new Happy Joy Plaque instead, making it unobtainable.
The Rollback Plaque was able to drop a seed as soon it was first released, however, this was not intended and was later removed, making existing seeds extremely rare.
The second rollback occurred on October 5th of 2013. Only a few hours of data was lost.
The issue which ruined the game this time was with special "Null_seed". The Null Seed was able to be obtained from using a Harvest Moon Blast where a Null Seed could've possibly be planted. The item would recycle for random, abnormal amount of gems, ranging from millions to negative millions of gems. The item existed long before they were discovered, however, the damage to the economy had already been done and a rollback was required to fix the issue.
The Rollback Plaque II was available during the second Apology Weekend, and was made by splicing a Cave Background Seed with a Sign Seed. Unlike the previous plaque, this recipe was not reused for another item afterwards, until the Rollback Plaque III came out.
The Rollback Plaque II Seed was able to be obtained during the event only, but, stopped dropping seeds once the event was over.
A third rollback occurred on August 9th of 2015.
The RAID arrays which were used to store data about worlds had fried, leaving corrupted and missing data. Since the data was missing, a rollback occurred to restore data to the last daily backup.
The Rollback Plaque III was available during the third Apology Weekend (August 10 to August 11), made by splicing Cave Background Seed with a Sign Seed. The item did not drop seeds and the recipe was not reused.
A fourth rollback occurred on December 12th of 2017. A security exploit was used to gain unauthorized access to developers @Seth and @Hamumu's accounts. Valuable items were unfairly distributed throughout the game and world data was corrupted, leading to the decision to rollback to the 10th of December, 2017. At the end of the Rollback, an Apology Event was held to help catch up on lost progress, any gem purchases made in the time period where time was rolled-back would be doubled as compensation.