The AnnotationComboIterator helper class used by the UIMA POSTagger accesses its iterators unsafely.
The consequence is that the AE breaks even on very simple CASes such as the CAS showcased on this repository (text of 9 letters, 2 sentence annotations and 9 token annotations): https://github.com/m09/postagger-iterator-bug/blob/master/in.xmi
The repository linked above contains an example program that crashes on my setup. It's fully maven 3 aware so you can normally launch it quite easily.
Here is a patch that should address the issue: https://raw.githubusercontent.com/m09/postagger-iterator-bug/master/iterator.patch