Running "Searge" as the command in the command block set the previous output as "#itzlipofutzli"

Hi

This is an intended easter egg 🙂

I know that but technically it should report "Unknown command at position 0: <--[HERE]" after 17w45a

This is intended, because it is an easter egg. 🙂

Code Analysis
The following is based on decompiled version of Minecraft 1.12 using MCP 9.40. All method and class names are the names used in the decompiled version.
The reason why this happens is because the method net.minecraft.tileentity.TileEntityCommandBlock.java has codes to detect whether the player has entered "Searge". See below.

public boolean trigger(World worldIn)
    {
        if (!worldIn.isRemote && worldIn.getTotalWorldTime() != this.field_193041_b)
        {
            if ("Searge".equalsIgnoreCase(this.commandStored))
            {
                this.lastOutput = new TextComponentString("#itzlipofutzli");
                this.successCount = 1;
                return true;
            }
        ...
        }
    }

public boolean trigger(World worldIn)
    {
        if (!worldIn.isRemote && worldIn.getTotalWorldTime() != this.field_193041_b)
        {
            if ("Searge".equalsIgnoreCase(this.commandStored))
            {
                this.lastOutput = new TextComponentString("#itzlipofutzli");
                this.successCount = 1;
                return true;
            }
        ...
        }
    }