I typed in upper case I (as in India) in the first LUA script and
lower case l in in (lima bean) in the second LUA script.

I typed in upper case I as in India in the first line
and lower case l as in lima bean in the second line

I ran Lua 5.1.1 on my computer and got the same bug.

I downloaded Lua 5.2.1 and tried the thing again and it was gone on Lua version 5.2.1.
What version of Lua currently runs with FoldIt?
Can I specify what version of Lua that runs with Foldit on my machine?

---------

Lua 5.2.1 Copyright (C) 1994-2012 Lua.org, PUC-Rio
> print( os.date() )
01/13/14 13:14:35
> print( os.date( "%I" ) )
01
> print( os.date( "%l" ) )
stdin:1: bad argument #1 to 'date' (invalid conversion specifier '%l')
stack traceback:
[C]: in function 'date'
stdin:1: in main chunk
[C]: in ?
>

My input comment screen had a font of New Courrier so that I as in India was easily
distinguishable form l as in lima bean.
This time I clicked on "Preview" and the output had a font of New Courrier.
Will the text on the FoldIt Feedback screen be in New Courrier so that I and l are easy
to tell apart?

This crashes FoldIt in the newchapter version too.

This web site has code for calling a Lua program from a C program with
a bail() function in the C program that should catch Lua errors.

http://www.troubleshooters.com/codecorn/lua/lua_c_calls_lua.htm

It's interesting that you found stock Lua libraries to crash. Foldit has a custom wrapper around print to add spaces between arguments, I would expect some error checking would need to be added to Foldit's wrapper. Perhaps it still does, if the libraries have an error.

The bug is not in Lua's print() it is in Lua's os.date( "%Q" ). Calling os.date() with an invalid
argument causes it. I ran it using Lua's version 5.1 Windows compiler and got the bug too.
I upgraded to the
latest version of Lua Windows Compiler which is 5.2.1 and the os.date() bug is fixed. The current release of Foldit
and NewChapter Foldit use Lua version 5.1.

I wrote a Lua script that would divide by zero. It crashed Foldit too.

Output from Lua Windows Compiler:

Lua 5.2.1 Copyright (C) 1994-2012 Lua.org, PUC-Rio

> x_str = os.date( "%X" )
> print( x_str )
10:29:07
> x_str = os.date( )
> print( x_str )
01/22/14 10:29:53
> x_str = os.date( "%Q" )
stdin:1: bad argument #1 to 'date' (invalid conversion specifier '%Q')
stack traceback:
[C]: in function 'date'
stdin:1: in main chunk
[C]: in ?
>
> print( "Lua version: ", _VERSION )
Lua version: Lua 5.2
>

The software release,
20140811-46cd6bb2fb-win_x86-devprev,
Fixed these bugs.

Lua Version: Lua 5.1
Foldit run on this date: 08/11/14 15:29:58
Valid table index: 902
Invalid table index: nil
Valid argument for Lua os.date() works 15:29:58
Doing divide with non-zero divisor: 0.33333333333333
Attempting divide with zero divisor: 1.#INF
Invalid argument for Lua os.date() 15:29:58

http://fold.it/portal/recipe/47844

a recipe called “version print” has been created to test these functions

There is no indication as to where the error occurred. Lua is compiled into a series of function calls when executed. Something like “Divide by zero in function check_helix + 128 “ would be nice.