Everybody in the Software Industrie knows there is nothing like a bug free software, which is alright if you deal with it in a reasonable manner.
Talk about it, inform your customers, prepare workarounds for the nasty ones and bugfixes/patches for everything else.
Oracle knows all this and provides a list of known bugs/issues with Oracle APEX 4.0.
If you encounter a bug which is not listed there, please report it in the forum (maybe using “Bug” or “Bugreport” in the Subject).
Don’t be shy, even if it turns out not to be a bug, you’ll definitely learn something :-)
Look at a very good example of how fast a bug can be solved as soon as it is reported.
For the current Patchset APEX 4.0.2. there is also a list of known bugs