From Software has identified a serious bug that has the potential to impede progression in their recently released title, Bloodborne. The bug can cause you to miss out on a boss fight. Fortunately, because the developer has identified the bug, it intends to put out a patch to address the situation fairly soon.
You can avoid the bug altogether by not initiating a multiplayer session in Bloodborne's Forbidden Woods area. So while you're in the area, don't use either the Small Resonant Bell or Sinister Resonant Bell items.
Spoilers ahead:
From Software has confirmed that the Lunarium Key, which is normally found in Byrgenwerth, does not appear because of a multiplayer bug. Without the key, it's impossible to visit Master Willem, the Moonside Lake, and Rom the Vacuous Spider. To visit these encounters, do not initiate multiplayer mode or you'll miss out on them.
From Software says that a hotfix for Bloodborne will fix the issue when it's released sometime this week.
The hotfix is separate from the title update they have planned for the game, which comes with improvements to load times and performance.
