I failed an FBI poly a few years ago for unknown reasons. Will this failed poly cause me not to become a Navy Seal. For obvious reasons the Seals need a certain security clearance but I am unsure if this failed poly will hindure me or not.

Having served for onver 20 years in the US military, with most of those in Special Operations and SOF support assignments, as well as training and experience as an SSO (Special Security Manger - managing cleared personnel and clearances)  I will tell you that your failure on the FBI poly will have very little bearing in the final adjudication of your clearance.  Your assignment in the Army as a reservist will be limited to what type of classified information you will gain access too.  Those that are granted TS-SCI clearance rarely come into contact with "live" information and those that do, when subject to a polygraph will be given the CI-Scope version.  The tactics in questioning are standard and right down to the T as described in TTBTLD, right down to "is there anything bothering you about one of these questions."  With a little practice you will even be able to predict the presise moment that the question will be asked.  As for those applying for SF/SEAL positions, you will not be required to have a TS unless you are being assigned to a command position within an SF Group headquarters (Gp Cdr, CSM, ect), or being assigned to the 1st SFOD-D (The "infamous" Delta Force, which by the way DOES exist and is admitted.)  Candidates who are selected for assignment to Delta are already SF soldiers with at leat 3 years on a team.  Those recruited off of the street or from the mainstream Army are very rarely "selected" for an SFOD-D assignment.  You have nothing to worry about.