Title pretty much says it all, but...
If for whatever reason Minecraft quits or stops, for example a crash, or a computer shutdown due to low battery, and you have items in the crafting grid of the crafting table, those items will be deleted upon relaunch.
(Hint-hint: Force a debug crash to reproduce without abnormal circumstances.)
Comments 4
That's why I use notepad as an example. Some software (unlike notepad) will make periodic backups, allowing the restoration of previous work. That being said, it would be impractical to do the same with Minecraft, as the game would suffer from severe lag, since these 'backups' (essentially game saves) would have to occur frequently.
Actually, Minecraft does save pretty frequently. Whenever a chunk is unloaded, it must be saved first. Whenever a player logs out, their .dat file must be saved. It would probably be bad if Minecraft did attempt to save data during a crash, because there's the potential to corrupt your world in the process.
However, the problem being encountered here is that the player's inventory and the chunks are stored in separate files, and there's no attempt to ensure that they are synchronized. Thus, under some circumstances, it's possible to lose or dupe items. In this case, the problem is that Minecraft is still saving the player's .dat file, but not the world. It should either save both, or save neither.
In order to correct this problem, they would need some kind of transaction synchronization, or a backup, so that either both the current chunk and the player's inventory are updated, or neither one is.
When Minecraft closes unexpectedly (eg. game/computer crash) it doesn't have time to save the data, and thus it it lost. It's the same thing as opening a notepad document, typing stuff, then killing the process: the data isn't saved. Unfortunately not much can be done about this.