I recently started playing a Merfolk deck, and I noticed a few cards that have bugs (as long as I am not interpreting the rules incorrectly).
With the card
Merrow Reejerey, its power activates correctly when a Merfolk card is put into play and un/taps the permanent you select, but then it deletes the merfolk card that you were playing in the first place to activate it. The card is not in
Exile, the grave, or returned to the deck.
The same thing is also happening to me with
Drowner of Secrets and
Drowner Initiate, although they have powers that are not exactly the same.
The game will not let me play
Belltower Sphinx even though I have more than enough land to bring it into play.
Enclave Cryptologist plays well the first time you put it into play, but then does not allow another copy of it to be played. It also sometimes duplicates the card that it draws from the card that was drawn on the draw phase. This duplicate is usually bugged and unplayable.
Edit: Two other minor bug's that I just found are the explanation text in the left hand text box when
Merfolk Sovereign gives unblockable to a creature, the text says creature "gains HIDDEN unblockable until end of turn."
Also dealing with the same card, the +1/+1 of
Merfolk Sovereign is not counted when you are on your main phase unless there is something that triggers a change in stats. The game does count the bonus +1/+1 correctly when entering other phases.
Edit 2: The enchantment
Briar Patch does not give -1/-0 to attacking creature correctly, if you have six creatures attacking, it gives six -1/-0 to a random creature that is attacking.