JDK 1.4 - 1.5 Compatibility Problem

It has been the third time, that I experienced problems by migrating a Project from JDK 1.4 to JDK 1.5. Every time I remembered about "a problem" but never remembered what it exactly was.
In this Post I want to sum up these Problems.
Actually I only know one, but it's noteworthy. REALLY :-)
The Problem causes Bigdecimal to work in a different way when using the toString method
Here is the bug report.

Blackdown Java vs. Sun Java

Found an interesting "bug" (it isn't a real bug, but choose yourself): If you have a Date (no matter if java.sql.Date or java.util.Date) in Daylight-saving Time before 1981 Blackdown chooses the historical way (no Daylight saving Time between 1948 and 1980) and Sun does ignore this historical point.
This isn't a real problem until you mix up the two distributions (eg. client-server application)