The fuel tag has a maximum value of 32767. Since each piece of coal you power it with increases the fuel tag by 3600, giving it 10 or more coal will cause the tag to wrap around into the negatives and never decrease anymore, essentially giving it infinite fuel. If you then give it 9 more fuel, it will become positive again, simply wasting the last 20 coal you gave it.
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Yes, but it means that is a player tries to use more than 9 coal, the minecart will go forever. That's obviously a bug. The tag should just have a limit, so any additional coal won't add to the time.
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15w36c This may be working by design, but a design that needs to be fixed!
@unknown it won't be infinite. When it is negative it will count to -32768 and then jump to 32767. However when increasing the fuel while it is negative, it becomes positive which means that you than lost at least 32768 ticks of fuel
Should be trivial to check the current fuel value and not exceed the limit. It's reasonable for a player to attempt to load up a minecart with furnace with a supply of fuel (assuming that it's reasonable for anyone to use a minecart with furnace at all), and it's not difficult to exceed the limit.
"Working by Design" : "fuel" is a short.
http://minecraft.gamepedia.com/Chunk_format#MinecartFurnace