Weird error: two similar strings are not equal

[Valeriy Skurikhin <keen.vs@...>]

Hello, list!

While looking into one nasty bug (was posted here
http://lua-list.2524044.n2.nabble.com/Strange-intermittent-LuaJIT-errors-likely-my-fault-td7438216.html)
I encountered curious error

./luajit: ../../etc/replay.lua:112: return values check failed: load 00000131
 -- return value #2 mismatch: actual `""', expected `""'
 -- return value #4 mismatch: actual `"luatexts"', expected `"luatexts"'
stack traceback:
    [C]: in function 'error'
    ./lua-nucleo/ensure.lua:329: in function 'ensure_returns'
    ../../etc/replay.lua:112: in main chunk

This happens during running the test, that reveals the bug I mentioned
above, so any kind of things may happen.  But I wonder, how can it
ever be, that two similar strings are not equal.  The test uses
standard comparator for string.  I'm not an expert in Lua, so it look
weird to me.  Any ideas are welcome.

Thanks,
Valeriy