I have two users for whom a call to (gl:get-integer :num-extensions) errors with:
OpenGL signaled (1280 . iNVLID-ENUM) from GET-INTGER-V

The only thing they both have in common it seems is OSX
Here is copied from one of their slime error buffers.

OpenGL signalled (1280 . INVALID-ENUM) from GET-INTEGER-V.
   [Condition of type CL-OPENGL-BINDINGS:OPENGL-

ERROR]

Restarts:
 0: [CONTINUE] Continue
 1: [RETRY] Retry SLIME REPL evaluation request.
 2: [*ABORT] Return to SLIME's top level.
 3: [ABORT] Abort thread (#<THREAD "repl-thread" RUNNING {1003448023}>)

Backtrace:
  0: (CL-OPENGL-BINDINGS:CHECK-ERROR #<unavailable argument>)
  1: (CL-OPENGL:GET-INTEGER :NUM-EXTENSIONS 1)
  2: (CEPL::GET-GL-EXTENSIONS)
  3: (CEPL::CEPL-POST-CONTEXT-INITIALIZE)
  4: (CEPL:REPL 640 480)
  5: (SB-INT:SIMPLE-EVAL-IN-LEXENV (CEPL:REPL) #<NULL-LEXENV>)
  6: (EVAL (CEPL:REPL))
  7: (SWANK::EVAL-REGION "(cepl:repl) ..)

It seems from here https://developer.apple.com/library/mac/releasenotes/general/macosxlionapidiffs/OpenGL.html that this enum was only added in OsX 10.7 (Lion) is that the most likely cause of the issue?

Baggers