1. Start with a fairly stock Rakdos Vampires Explorer deck (see screenshot for list). Switch any two card -: one from the top-seven and the other from the bottom-eight portions of the sideboard - and the 4 Leylines at the bottom of the list will replicate. The total cards in sideboard will still say "15," but it will show those 15 PLUS FOUR MORE Leylines (8 total).
   

Switch any two cards between top and bottom portions again, and the list will now show over 100 copies of Leyline of the Void (about 114; I may have lost count).

Switch a third time, and the extra copies will disappear.

Clicking on ANY copy of Leyline will remove all the extras, plus one actual card from the sideboard, down to 14 total cards.

The above results vary slightly if one of the swapped cards is a Leyline of the Void; duplicated Leylines appear in each section, and caused a significant lag in the game.

This bug occurs in either deck-display mode.

1.   I am using a PC.
   

2.   The above deck list and aforementioned card swap creates duplicate, uncounted copies of Leyline of the Void -- four extra copies after the first swap, over 100 extra after the second.  If a Leyline of the Void is one of the swapped cards, extras are created in each portion of the sideboard.  Sideboard card count remains at 15.  Clicking on any copy of Leyline of the Void removes the extras, plus one actual.
   

3.   I would not expect extra, uncounted copies of any cards to appear in my sideboard, ever.  
   

4.   See attached screenshots and system log, with titles that hopefully explain what you're looking at. In the third image, showing a full screen of Leylines, note the position of the scroll bar on the right.  There are a LOT more!
   

5.   This bug does not occur in my other deck lists.  I will post a reply to this thread if I observe any similar issues, in the hopes of narrowing down the cause.