This sounds legit. Anyone mess with the if node recently? There seems to be a regression where existing content is broken. This should be fixed ASAP and patched into 4.6.

Looks to be caused by this

Change 2341146 by Andrew.B on 2014/10/27 11:14:16
Prevent Const material parameter values from crashing
#jira [Link Removed] : LIVE: EDITOR: MATERIALS: CRASH: Setting the MipValueMode for MipLevel or MipBias to 4 or higher will crash the editor
#change Modifiied those Const values to be ran through the compiler than used as is (three occurances that I could see).

int32 Arg4 = AEqualsB.Expression ? AEqualsB.Compile(Compiler) : ConstAEqualsB;

int32 Arg4 = AEqualsB.Expression ? AEqualsB.Compile(Compiler) : Compiler->Constant(ConstAEqualsB);

ConstAEqualsB is

float ConstAEqualsB;
ConstAEqualsB = INDEX_NONE;

The hlsl translator does

virtual int32 If(int32 A,int32 B,int32 AGreaterThanB,int32 AEqualsB,int32 ALessThanB,int32 ThresholdArg) override

if (AEqualsB != INDEX_NONE)

{ ... }
else{ ... }

So it used to never take the top branch, unless an artist modified ConstAEqualsB. Now it always takes the top branch.

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ComponentUE - Rendering
Affects Versions4.6
Target Fix4.6.1
Fix Commit2384073
CreatedDec 9, 2014
ResolvedDec 10, 2014
UpdatedApr 27, 2018