Ok, my basis for saying “most common” is not at all scientific, just a gut sense of how many times I’ve seen 15 mins or more spent trying to find a bug that ended up being just a syntax error in one’s script. Tools like JSLint and Firebug are really helpful in general, but not with these issues. (I’ve heard Aptana should be, but I couldn’t get it to work and it kept crashing on me.)

1. Spurious comma after the last entry in a JSON object string
   

   {name:’Fred’,visits:3,}

2. Use of = instead of : in a JSON object string
   

   {name=‘Fred’}

3. Misspelled function and object names
   

   encodeURICompoent

   onReadyStateChange instead of onreadystatechange

4. Spurious parenthesis after a function call
   

   confirm(‘Proceed?’));

5. Using parenthesis instead of braces for access within arrays and objects
   

   htsVisits(‘Fred’)

6. Spurious : between keyword case and a switch value
   

   case: 1:

7. Expecting “False” or new Boolean(“False”) to evaluate as true