Nibiru
There are tons of people pushing Nibiru, claiming it can be seen at some times, and not at other times. Planets do not just show up and vanish. Either it is there or it is not.

There is a trick you can play on children up to about four years of age. It is the "I took the cookie you just ate" trick. If the kid is eating small cookies, like Chips ahoy or Oreos, wait for the kid to finish one. Then, put a cookie in your hand and put your hand on the kid's tummy and rub it for a second, then take your hand away and show the kid the cookie, while proclaiming "I just took the cookie you ate". The kid will believe you and demand it back. The same kind of cookie trick is going on with Nibiru. However, no amount of rubbing your tummy is going to make Nibiru show up after the sun has set and there is no lens flare.

OPINION: Nibiru is possible. There is no reason why the solar system could not have a huge dark planet on a gigantic orbit that completes in thousands of years. But once it becomes visible, it is not just going to come and go like a ghost. It will be ever present for all to see, until it passes the way a comet does.
--Mexican Anonymous