For better understanding of the issue there is a wireshark capture attached, use "View->Time Display Format->Seconds Since Previous Displayed Packet" for a better view of the timings. The script creates monitored items, modifies the subscription and then sends two WriteRequest to provoke new values being sampled in the items. The nodes the script uses are of datatype boolean and byte, the WriteRequests (in the capture these are the last two captured WriteRequests) first write False and afterwards True to the boolean node, thus restoring the initial value. The delay between these WriteRequests varies a few milliseconds every run and sometimes is below the sampling interval (50 ms) like 44 ms in the attached wireshark capture. In rare cases this causes the item to not queue a new value for the boolean as it is only changed for a short moment between taking 2 samples, causing the script to fail.