Tracefile
In the GnuCash application, all error messages during the program run are redirected into a trace file. The content of this file is very important when reporting a bug, as well as a Stack Trace for bugs that crash the program.
At each program start, a new trace file will be written and all output during the program run will be appended to that file.
Trace file on Windows
On Windows, these files are located in the folder c:\Documents and Settings\myname\Local Settings\Temp\gnucash.trace.ABCDEF for a user named "myname" (on Vista in C:\Users\myname\AppData\Local\Temp), where the .ABCDEF suffix is changed randomly on each start of gnucash. This location may change, if you set the Windows environment variables TMPDIR, TMP, or TEMP (in DOS mode, use the SET command). Also, the folder name "Local Settings" might be named differently in your native language (e.g. in German "Anwendungsdaten"), and it might also be a "hidden system folder" which is shown only if you change the folder properties to "Not hide system files/folders".
If you experience crashes or unexpected behavior, check the latest of these files (or the one created when the problem started) and add all interesting-looking lines of that file to your error reports (e.g. in Bugzilla). Please include at least the lines with the keywords WARN (for warning) and CRIT (for critical warning).
Trace file on Linux
On Linux, these files are located in /tmp.