What is an update error ?
When an update fails to happen, SAP sends an express email saying the update was terminated with contents similar to this

System ID.... D82
Client....... 500
User..... XXXXXX
Transaction.. VA02
Update key... 44DB4B8B86D81213E1000000A
Generated.... 14.08.2006, 11:38:43
Completed.... 14.08.2006, 11:38:44
Error info... TD 423: WRITE_FORM

A pop-up will also be displayed updating the user about an email sent to his inbox with the above data.
There are 2 ways to diagonize this.
1. SM13 (to analyze/view update requests)
specify the user id and the list of pending/failed updates will be shown.
check mark the line that suits the situation based on time-stamp or type of transaction or double click on the line.
double click on the errored out line.
click on the abap-code link as shown in graphic above. It will take you to the ABAP editor and point out exactly a couple of lines after the error has occured.
2. ST22 (to analyse core dumps)