Enter the following script into the debug console of a mission and press LOCAL EXEC. Note the lack of a closing curly brace on the spawn block.

[] spawn
{
    while { true } do
    {
        systemchat format ["Hello %1", diag_tickTime];
        sleep 1;
    }

Add the closing curly brace, press LOCAL_EXEC and the script will spawn.

Misspell "systemchat", press LOCAL EXEC and the compiler will report an error.

The first time this happened to me, I was stuck for hours. I never considered that a compiler wouldn't report a basic syntax error.

Note that the author of a large mission or mod can have a terrible time figuring out what went wrong. Symbols will simply be undefined, and the scripting language is pretty tolerant of undefined symbols, meaning even less information to go by.