If you encounter an error like
ERROR 1: Error returned by server :

java.lang.IllegalArgumentException: argument type mismatch
argument type mismatch



you probably have a mismatch between WFS versions. Try changing your WFS url to a specific version. Using http://www.example.com/geoserver/sf/ows?strict=true&version=1.0.0 worked for me after I spent hours with that stupid error…

If you add features to your store but nothing new appears on the map, check if the features actually end up with geometries on your server… Eg if you try to add LineStrings to a MultiLineString layer via Geoserver WFS-T you will get objects without geometries. This happens silently, at least when using the osgeo ogr module in Python.

If every request takes 10 seconds your server might time out trying to parse your schema. Check your geoserver logs for things like
2016-03-10 15:48:00,486 WARN [geotools.xml] - Error parsing: http://123.123.123.123.:8080/geoserver/ows?strict=true&VERSION=1.0.0&SERVICE=WFS&REQUEST=DescribeFeatureType&TYPENAME=my:layer

If you know how to find out how to debug this please tell me:
2016-03-10 15:48:00,501 ERROR [geoserver.wfs] - Transaction failed
org.geoserver.wfs.WFSTransactionException: Error performing insert: null
at org.geoserver.wfs.InsertElementHandler.execute(InsertElementHandler.java:215)
at org.geoserver.wfs.Transaction.execute(Transaction.java:322)
(... a million more steps truncated...)