Before that patch, making an emergency call on some networks (e.g. Rogers in Canada, and T-Mobile in the USA) would cause a system service to throw a NullPointerException, and the phone to reboot.
I am really quite surprised and shocked. How on earth could such a bug — in the single most important feature a phone has, a malfunction in which could be life-threatening — make it into multiple models of phone without detection until after release? It's hard to say precisely who is at fault here, so I'll go with "everybody". The network, the manufacturer and the OS developer should all be clear to themselves and to each other who is going to make sure that this stuff is actually fit for purpose. Android 1.5 clearly wasn't suitable for use in a phone, and given the lack of evidence of any significant change in behaviour of Google/HTC/... since then I have no confidence in any version of Android. Maybe as a netbook OS it would be great, but I just don't want to potentially trust my life to it.
(In the UK, DECT phones very sensibly carry a warning that they are not to be relied upon for emergency calls and that alternate provision should be made. Maybe Android phones should carry a similar warning, suggesting that one also carries a non-Android phone for emergency use!)
Incidentally, I don't think that this has resulted as a consequence of Android's open source nature: having prodded git a bit, I believe the bug was present in a bulk Google upload on 3rd March 2009. Someone more familiar with the Android source control maze might be able to confirm that.