BizTalk - Most possible reason for “is Delimiters are not unique” in EDI transaction

I have one EDI to IDOC map deployed on BizTalk2006, and after I dropped an EDI file into the receive location, I got the following error.

Fatal error encountered in EDI Disassembler, error information is Delimiters are not unique, field and component seperator are the same

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From the error description, I thought there were multiple delimiters within the input data, After I  double checked my input data, I realized that’s not the reason.After comparing with other data files which can pass the schema validation, I found any field level violation in envelop can cause this type of error. E.g. ISA09 is a date format, but the value here is seven digitals, in my case the ISA06 field has extra white spaces and max length exceeds 15.