The first thing which you need to check is ‘Type of Error‘. Without knowing the type you can not investigate. During the performance test, you may get different types of error like client-side error, server-side error etc.
Scriptings Errors:
a). Missed to replace old URLs in some places: This causes the failure of a particular transaction.
b). Lack of test data: Sometimes tester forgets to set “recycle the test data list” or “continue with last value”
c). LG failed during the test: This issue occurs when LG(s) are unable to handle user load during the test.
d). LG memory issue: This issue comes when there is no enough memory in LG. LG may get disconnected in between the test.
e). HTTP 4XX: Refer response code graph, if the test result having HTTP 4XX error then this indicates something wrong has been sent to the server.
HTTP 4XX followed by 5XX: Again this is a syntax of client-side error, but not always. This is a tricky scenario where you have to carefully analyse the previous requests having 4XX response code and the following request having 5XX response code.