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Describe the Bug:
When a creature enchanted with Sigil of Sleep deals combat damage Sigil of Sleep triggers and you can select a creature that has been dealt lethal damage.

Which card did behave improperly ?
Sigil of Sleep

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
Manalink 2016/01/15: Battle For Zendikar v2,duel

What exactly should be the correct behavior/interaction ?
Sigil of Sleep effect can't return to hand a creature that has been dealt lethal damage.

Are any other cards possibly affected by this bug ?
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I strongly suspect, without having the time to test it right away, that it does take place after damage is assigned like it should, but before the damage prevention step. That's going to make it very difficult to reproduce the proper behavior under modern rules.

In fact the trigger resolves after damage prevention step but before regeneration step and till then objects are not yet considered to have been dealt lethal damage.