The introducer protocol fails if

• The two emissaries think they are talking to the same introducer.
• They are talking to distinct introducers, perhaps in separate nodes.
• A rogue, having intercepted the indexes, puts himself in the middle acting as the counter-party at both introducers.

If an introducer, already categorically trusted, has a serial number that is unique among introducers, this will fix the problem but it puts a peculiar burden on the code that initiates the introducers to ensure they have distinct serial numbers.