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Hi jestersheepy,
Have you logged an Answerhub bug report for this? If so, please link it here. In the Answerhub report, please share the full callstack of the crash you are seeing, and any useful information about the type and contents of your blueprint.
Thanks
UPDATE: I assume the crash you are experiencing may be the same as reported here: 4.12.1 Crash when trying to compile (DEMO PROJECT) - Programming & Scripting - Epic Developer Community Forums
This was inadvertently introduced in 4.12.1, and we are currently working to resolve this in a new hotfix as soon as possible.
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I will see if I can debug it more in a bit, at first glance it is crashing on compile of the majority of my blueprints that are placeable (in world actors). Thats quite vague though 
Also not sure about this but the material editor seems to be compiling far more shaders and more frequently than before?