You know when you put a ticket through a barrier on the Underground and 
"Seek Assistance" comes up on the display. Well, if you look on the 
left- hand side of the display unit when that happens, you'll see a two-
digit code which the member of staff uses to work out what's wrong with 
your ticket.   

Note: On the newer gates, the code is displayed on the main barrier 
head and can, afaik, be seen from either side just by looking 
underneath the "Seek Assistance". This also applies to the Mark III and 
Mark IV gates but not, for some reason, to the far-left set of Mark IV 
gates (the Prestige ones).  

And now, here's the complete list of codes and their meanings, courtesy 
of an LUL employee who wishes to remain anonymous.  

00 - Valid Ticket
01 - UTS Gate Fault (Invalid Tables)
02 - Wrong Type/Direction of use for this type of ticket
03 - BR only - no LUL validity
04 - UTS Gate Fault (Check Table Error)
05 - Technician signed onto machine
06 - Test Ticket
07 - Code Unreadable (Usually when ticket is upside down)
08 - Recoding Error
09 - Ticket Damaged
10 - Carnet Validated (Carnet Validator Only)
11 - Out of Date
12 - Not Valid Now (e.g.. OPTC before 0930, Freedom pass before 0900)
13 - Additional Fare Payable
14 - Not Valid, and is not excessable
15 - No onward validity
16 - Pass at Invalid station
17 - Excess/platform ticket at wrong station
18 - Station Permit at wrong station
19 - Start Date in future
(20 - Spare)
21 - Ticket already used for entry
22 - Ticket already used for exit
23 - Carnet not validated on entry
(24-30 - Spare)
31 - Invalid From OSI
32 - Invalid Into OSI
33 - Illogical Interchange through gates [1]
(34-40 - Spare)
41 - Ticket used three times in quick succession at same station
(entry-exit-entry or exit-entry-exit or purchase-entry-exit)
42 - Double use in one direction
(43-50 - Spare)
51 - Already used for 1 journey (single) or 2 journeys (return)
52 - Pass Reported Lost or Stolen [2]
53 - Permit Reported Lost or Stolen [2]
54 - Possible Dumbbell
(55-60 - Spare)
61 - Too long spent making interchange
62 - Too long spent on journey (carnets only)
63 - Too long on platform (platform tickets only)
64 - Too long on excess (Fare-Paid tickets only)
65 - Entry time exceeded on platform ticket
(66-72 - Spare)
73 - Ticket given back for possible emergency use (e.g.. substitute bus
service) and is not valid at this station
(74-81 - Spare)
82 + 83 - Illogical Use of Ticket
(84-89 - Spare)
90 - Gates set to reject this ticket type (normally child-lock)
(91-98 - Spare)
99 - UTS Gate Fault (Check Time-out)

If I've missed any, please let us know.

These codes come into four catagories :

(1) - Those in everyday use - 00, 03, 07, 08, 09, 11, 12, 13, 21, 22,
      41, 42, 51, 90
(2) - Those used often in the city, but not on SA's list - 10, 23,
(3) - Others on the SA's list - 02, 05, 06, 16, 17, 18, 19, 33, 52*,
      53*, 61, 62, 63, 64, 65, 73, 82, 83,
(4) - Only in technical material - 01, 04, 14, 15, 31, 32, 54*, 99
(*) - Not Used - 20, 24-30, 34-40, 43-50, 55-60, 66-72, 74-81, 84-89,
      91-98

NB: The software for this was very unreliable, and so has not happened
    for years (if ever, if I'm understanding the situation correctly).
    Affects those with *.

[1] #33 is when, say you exit through Monument station and re-enter at 
Tower Hill.  The interchange is illogical, so won't accept your ticket. 
 

[2] Pass is things like Staff/Dependant/Retired passes. Permits are 
things like Freedom Passes.   

Both can be intercepted if reported lost or stolen (retained by 
machine, machine closes awaiting staff assistance.  Code remains for 
staff to decipher)   

Note: RCI = Revenue Control Inspector, PFN = Penalty Fare, SA = Station 
Assistant, RCM = Revenue Control Manager(?)  

NB: The above codes also apply on National Railways barriers, which are
virtually identical to the LU ones.