Happened on my modded game when I updated Forge via the Curse Client. Attempted to reproduce using only LiteLoader and Forge successfully.

Full log: https://pastebin.com/KKSceydn
 

The important bit:

Spoiler

[00:45:35] [main/ERROR]: Unable to launch
java.lang.reflect.InvocationTargetException: null
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51]
    at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51]
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: joptsimple.MultipleArgumentsForOptionException: Found multiple arguments for option width, but you asked for only one
    at joptsimple.OptionSet.valueOf(OptionSet.java:179) ~[OptionSet.class:?]
    at net.minecraft.client.main.Main.main(SourceFile:81) ~[Main.class:?]
    ... 6 more

 

Note that Forge version 14.23.4.2759 is compatible.

See the existing thread.