Insane bug, here what you need to do:
1. Buy Massdriver (don't buy chainsaw);
2. Buy Suicide Bomber and switch quickly to Massdriver for 2-3 times so it doesn't switch you back to bomb;
3. Get hit under 40 HP so self-Medical attention (needle) is avaivable;
4. After self-heal you will automatically get back to bomb and you explode.

Here is my demo: http://s000.tinyupload.com/?file_id=652 ... 9981836020 (IDE I used for recording)

V2 (edit):
1. Buy Massdriver and chainsaw;
2. Purchase Suicide bomber and switch to Massdriver repeatedly (2-3 times), the same step.
3. Sell chainsaw to get bomb pack and then explode.