Setup: Had ~70 Elf tokens using Imperious Perfect for most of match and then Elvish Promenade to inflate token count. Had three Heedless Ones at ~90/90 and a board full of other elves. Also had Might of the Masses in my hand which I was waiting to use for my final attack.
CPU had ~3 cards left in library, I had a few more.
1.Garruk cast Engulfing Slagwurm
2. I had two of whatever elf creature can be tapped to add 5/5 counter to target creature. Attacked with one, Garruk blocked with Engulfing Slagwurm and I tapped the other card of the same type to add the counter not realizing the Slagwurm would destroy it anyways and heal Garruk.
3. Garruk then attempted to attack with the Slagwurm.
Right before I could choose a blocker, the title crashed.
Seems like the crash occured while calculating the possibility of the buffed Heedless Ones blocking? Pretty upsetting considering I was planning on attacking with my first creature with a power of 150+ to end it.
Hope this helps you narrow down the cause.