Zynga says this happens when a connection between your computer and their servers are dropped.
"Falling out of sync can cause recent changes to your farm (prior to falling out of sync) to revert to their previous state. Don't worry however when you re-establish or refresh your connection your coins, cash, and/or items will all be back in your inventory, so while this is an inconvenience you are not losing items," says a Zynga customer service post.
We asked Zynga for some troubleshooting tips when you receive this error, and will update when we hear back.
In the meantime, you may want to try these general troubleshooting trips from the official FarmVille message board; they seem like they're worth a try if you get code 28 or have any other problems on your farm.
1. Try switching your browser. If you're using IE, try Firefox. If you're using Firefox try Google Chrome, etc.
2. Make sure you're running the latest version of Flash.
3. Try clearing the cache on your browser. Click here to learn how to clear your cache.
4. Please disable any ad blocking software that might interfere with the game.
5. Close programs running in the background.
6. Plug in -- wireless connections are more susceptible to breaking communication with servers.
If you have any other helpful tips for fixing the code 28 error on your farm, please comment below.