At last count MTG Forge has a total 1,841 cards and a number of errors. I personally know that some cards are more buggy than others because I programmed them. I tried my best to get Empty the Warrens and Reach of Branches working correctly but they are still buggier than Auntie May's blueberry pie in summer (very buggy).
Those two cards should probably be removed from MTG Forge because they don't work well. Other buggy cards have "state effects" (now called state-based actions) like Glorious Anthem, sliver cards, and Wonder which really slow down the game.
MTG Forge has many bugs because we try to add as many new cards as possible. I was very happy to be able to add planeswalkers. Their only flaw is if the computer has two planeswalkers in play, you can only attack one of them. While this is technically a bug, I still think it is acceptable considering how planeswalkers are generally very powerful.
MTG Forge does not have any kind of error reporting mechanism. If you have an error, such as if the program just says, "The computer is thinking" it means that the AI has crashed and the program doesn't know what to do. Ideally it would be good to save this information to a file, so we would exactly know the problem without the person having to explain the whole situation.
Even though MTG Forge can only implement relatively simple Magic cards, I'm still glad that it supports Baneslayer Angel, Akroma Angel of Wrath, first and double strike, exalted, wither, persist, protection, shroud and morph.
Feel free to report any errors to the forums. We have over 80 pages of errors!!! (The 80 pages represents a considerable length of time and hopefully most of the bugs have been fixed.) I heard that someone pumped Chameleon Colossus so much that its power became negative, which happens when a Java variable "overflows".
You can use the computer's activated abilities. I don't consider this a bug and I use it from time to time when the computer has an unfair advantage (aka when I'm losing).
The only "real" error reporting system is that I throw a RuntimeException if something shouldn't happen. Can this value be negative, if it can't, throw an exception. While this isn't perfect, it works "good enough" to allow me to fix the problem.
There is now a small team that works on maintaining MTG Forge, thanks to dennis, rob, and others who have helped.