We usually use SimpleLogger in tests, and some other binding when running in production.
In some cases, our tests depend on log values logged through the logger during a test.
Unfortunately between version 1.7.0 and 1.7.1 this ability has been removed, since the SimpleLogger now caches the system stream in a static initialiser.
Here is the code that broke this approach:
https://github.com/qos-ch/slf4j/commit/996ccc7ca507994465c069554954ecb01890d4d5

Basically, we would write a test:

PrintStream err = System.err;
PrintStream temp = new PrintStream(some buffer here);
System.err = temp;
// run tests

System.err = err;
// here we assert that the temp stream contains values we expect

Unfortunately SimpleLogger now caches the system stream and never releases it, meaning swapping the streams to catch the output is not possible.