Atualizei o XCode 5 para 6 para testar meus projetos no iPhone 6 e 6 Plus e estava recebendo falhas enigmáticas na inicialização:

'*** -[__NSArrayM objectAtIndex:]: index 4 beyond bounds [0 .. 3]'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010569b3f5 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x0000000104b20bb7 objc_exception_throw + 45
    2   CoreFoundation                      0x00000001055864d3 -[__NSArrayM objectAtIndex:] + 227
    3   UIKit                               0x00000001028164e1 _UIViewTopDownSubtreeTraversal + 193
    4   UIKit                               0x0000000102e43286 -[UIView(UIConstraintBasedLayout_EngineDelegate) _invalidateSystemLayoutSizeFittingSizeAtEngineDelegateLevel] + 128
    5   Foundation                          0x0000000103aa66df -[NSISEngine tryToAddConstraintWithMarker:expression:integralizationAdjustment:mutuallyExclusiveConstraints:] + 915
    6   Foundation                          0x0000000103c2f678 -[NSLayoutConstraint _addLoweredExpression:toEngine:integralizationAdjustment:lastLoweredConstantWasRounded:mutuallyExclusiveConstraints:] + 275
    7   Foundation                          0x0000000103a9ae3a -[NSLayoutConstraint _addToEngine:integralizationAdjustment:mutuallyExclusiveConstraints:] + 220
    8   UIKit                               0x0000000102e40495 __57-[UIView(AdditionalLayoutSupport) _switchToLayoutEngine:]_block_invoke_2 + 474
    9   Foundation                          0x0000000103aa8a8e -[NSISEngine withBehaviors:performModifications:] + 155
    10  UIKit                               0x0000000102e4029b __57-[UIView(AdditionalLayoutSupport) 

Após várias tentativas e erros e tentando vários XIBs em várias configurações, o problema ocorreu porque eu estava misturando alguns XIBs com o Auto Layout ativado, contendo outros UIViews (baseados em XIBs) com o AutoLayout desativado.

Desativar o AutoLayout para todos os meus XIBs resolveu o problema.