Well it depends on why the share prices are falling - there could a number of reasons why share prices may be going down and not all of them indicate a disastrous situation (or a situation in which share prices will remain low for a long period of time).
Therefore it is possible that if investment bank sees price fall as a short-term movement or something that is not supported by company fundamental business position to consider this a good opportunity.
Of course there is always a possibility to have a "deeper" interest in advising to invest in a particular issue which I'm not commenting here.
It is not clear whether your concern is on Enron or Lehman Brothers. Both cases attest to shortfalls of creative accounting. Enron's level 3 fair value accounting and repo 105/108 to Lehman Brothers. Both are cases of great corporate governance blunders in the use of derivatives.
Andersen was too optimistic with Enron's first time projects while Fuld was greedy about the mortgage market. Yes Bernanke could have rescued Lehman after Bear Stearns but then public money do have parameters.
hi - watch the great movie on youtube i've shown students - search "enron smartest guy in the room full movie" - it was that the investment banks and their analysts did not want to damage their relationship with enron, so were a bit loose with the truth. re the movie, grab some popcorn and enjoy!!
I think it is depend on the reason for fail the prices , some shares fail because the investors practice the speculating but the operating activities is good .