See the attached blend file. Ignore the script in the text editor. Simply go in the python console and type :

>>> z = bpy.data.objects['Z']
>>> z.vertex_groups[0].weight(33)

Blender crashes with the following backtrace :

Thread 0 Crashed: Dispatch queue: com.apple.main-thread
0 org.blenderfoundation.blender 0x00045d5f ED_vgroup_vert_weight + 207
1 org.blenderfoundation.blender 0x003b80f1 VertexGroup_weight_call + 49
2 org.blenderfoundation.blender 0x003550e2 RNA_function_call + 50
3 org.blenderfoundation.blender 0x006d10f7 pyrna_func_call + 807
4 org.blenderfoundation.blender 0x00b2c343 PyEval_EvalFrameEx + 27907
5 org.blenderfoundation.blender 0x00b2e2f3 PyEval_EvalCodeEx + 2211
6 org.blenderfoundation.blender 0x00b2e41f PyEval_EvalCode + 95
7 org.blenderfoundation.blender 0x00b21020 builtin_exec + 224
8 org.blenderfoundation.blender 0x00b2c343 PyEval_EvalFrameEx + 27907
9 org.blenderfoundation.blender 0x00b2ce9e PyEval_EvalFrameEx + 30814
10 org.blenderfoundation.blender 0x00b2e2f3 PyEval_EvalCodeEx + 2211
11 org.blenderfoundation.blender 0x00b2c2f8 PyEval_EvalFrameEx + 27832
12 org.blenderfoundation.blender 0x00b2ce9e PyEval_EvalFrameEx + 30814
13 org.blenderfoundation.blender 0x00b2ce9e PyEval_EvalFrameEx + 30814
14 org.blenderfoundation.blender 0x00b2e2f3 PyEval_EvalCodeEx + 2211
15 org.blenderfoundation.blender 0x00aad9c9 function_call + 169
16 org.blenderfoundation.blender 0x00a7dc89 PyObject_Call + 89
17 org.blenderfoundation.blender 0x006d4548 bpy_class_call + 680
18 org.blenderfoundation.blender 0x0040bd2c operator_execute + 156
19 org.blenderfoundation.blender 0x0001865c wm_operator_invoke + 284
20 org.blenderfoundation.blender 0x000194f1 wm_handler_operator_call + 737
21 org.blenderfoundation.blender 0x000196a8 wm_handlers_do + 392
22 org.blenderfoundation.blender 0x0001a44b wm_event_do_handlers + 923
23 org.blenderfoundation.blender 0x000148d0 WM_main + 32
24 org.blenderfoundation.blender 0x00014013 main + 3171
25 org.blenderfoundation.blender 0x00011512 _start + 216
26 org.blenderfoundation.blender 0x00011439 start + 41

The system is Mac OS X 10.6; the build is r35884 (32-bits) from Graphicall OR the RC0, both show the same issue.

Note that

>>> z.vertex_groups[0].weight(32)

works fine (all IDs from 0 to 32 are fine), even negative IDs work without crashing (not sure they should?), and also

>>> z.vertex_groups[0].weight(999)

works without crashing however. I have been unable to determine why certain IDs crash and others don't (but those who do crash consistently crash every time).