In the jungle that is Wall Street, even one stumble is not forgiven, and Comdisco Inc’s disastrous foray into the risk arbitrage business has left America’s premier computer leasing company wide open to a hostile bid. To minimise the danger, the Rosemont, Illinois company has adopted a shareholder rights plan under which one right will be distributed for each share held, entitling the holder to buy one new share at $100 in the event that a person or group acquire 20% or more of the company’s shares, or receives through a tender 30% or more. If a would-be acquirer gets over 40%, then rights held by other shareholders will be redeemable at just 20% fo the then current market price. Comdisco will also be entitled to redeem the rights at five cents a share any time up to 15 days after announcement that a 20% or 30% stake is held. The rights will expire on November 17, 1997. Comdisco shares are currently trading at a lowly $17.875.