It looks like tag change scripts are broken in 7.7 (this is with beta 4). No matter what code is in the script, i get a NullPointerException:
INFO | jvm 1 | 2014/06/26 18:22:53 | WARN [SerialExecutionQueue ] [18:22:53,571]: Uncaught Throwable during execution.
INFO | jvm 1 | 2014/06/26 18:22:53 | java.lang.NullPointerException
INFO | jvm 1 | 2014/06/26 18:22:53 | at org.python.core.PyJavaType.wrapJavaObject(PyJavaType.java:85)
INFO | jvm 1 | 2014/06/26 18:22:53 | at com.inductiveautomation.ignition.common.script.TagChangeScriptExecutor$TagChangeExecutionCallback.execute(TagChangeScriptExecutor.java:163)
INFO | jvm 1 | 2014/06/26 18:22:53 | at com.inductiveautomation.ignition.common.script.TagChangeScriptExecutor$TagChangeExecutionCallback.execute(TagChangeScriptExecutor.java:125)
INFO | jvm 1 | 2014/06/26 18:22:53 | at com.inductiveautomation.ignition.common.util.SerialExecutionQueue$PollAndExecute.run(SerialExecutionQueue.java:99)
INFO | jvm 1 | 2014/06/26 18:22:53 | at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
INFO | jvm 1 | 2014/06/26 18:22:53 | at java.util.concurrent.FutureTask.run(FutureTask.java:266)
INFO | jvm 1 | 2014/06/26 18:22:53 | at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
INFO | jvm 1 | 2014/06/26 18:22:53 | at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
INFO | jvm 1 | 2014/06/26 18:22:53 | at java.lang.Thread.run(Thread.java:745)