Avatar: The Last Airbender (the show, mind you.)
They put the thing on hiatus for 6 months in the middle of it's last season without any explanation. Then one day, out of the blue, they say "Oh by the way, we'll be showing the last episodes next week." Ultimately, the last 5 episodes felt rushed and we didn't get to see Aang and Katara's relationship really blossom. We are just left to believe that it worked out in the end; a safe assumption, but one they should have shown more of.
Hope I didn't sound like a 13-year-old fangirl in that rant...