I have a Fire Magic, which can deal damage to all creatures.

My opponents board is (se log file)
2/4
2/4 (Same 1/3 with a counter as the 2/4 above)
3/4
4/3 (Bomb, opponent will not block with this creature)

I have a 4/3 (same bomb as my opponent), and enough mana to play a spell that kills any creature as well as the fire magic dealing 2 damage to all creatures.

My plan is:
If opponent only blocks with 2 creatures: Kill the biggest blocker. My creature will survive.
If opponent blocks with 3 creatures, kill the bomb, distribute 2 damage on each of the two biggest blockers and play fire magic dealing 2 damage more. This will leave my opponent with only a 2/4 left.

This happens:
Opponent blocks with the 3/4 and one 2/4. I play the kill spell on the 3/4 before damage as planned. We go to damage, and suddenly both 2/4 are now blocking! I could distribute 2 damage each and finish them off with my fire magic, but in this scenario I loose my bomb and my opponent still has his.

Blocking visualization needs to be clear on which creatures actually are blocking. Taking the wrong decision made me loose the game.