I will share a experience together with a recruits perspective about test of a candidate skills in debugging. I onced got on a interview that had three stages. The second stage was a "practical case". I didn't know more at that moment. While there I was informed there are a system that stopped working and they don't know. Some bugs is lying behind.
It was arranged as a remote desktop to a old testing environment. Probably to a unplugged or isolated environment. The project was a few webforms with some ASP.NET controls and related Code-file code. The codefile referred to a kind of business layer for which i just have a dll, no source code and method descriptions. The Webforms did the CRUD functions you can expect. Also a small search function. The business layer, in turn, talked to Views and SP in a sql server.
They broked some parts at different levels. I was given a paper with symptoms. "Can't search" "The 'region' field disappeared after last update" and such. Such as you can receive from your users.
I don't remember all details but at least a table field got renamed, which lead to a broken SP, which was used by the search function. That means no error in VS and no BL source code to trace field names. A SELECT parameter against Sqlcommand was misspelled and caused a webform to malfunction. Also a field was omitted which was the missing field in GridView (Autogeneratecolumns). A ASP.NET Button was referred to something who must meant to be a duplicated, enhanced, method and "forgot" to point button to new method.
Also such minor thing using title in a html tag that don't allow it. Also opposite ALT tag was omitted in a control that required it. There was also some errors with uncorrect closed html tags but which did not malfunction. Not sure if all those was a pure playhouse-project-error or perhaps same project for different recruitments. I never asked. The level of difficulty should of course match the need of the recruit.
Such test should probably be screened (not followed) to see, after interview, how debugging was done. For myself at that stage, I found the test a little ridiculous, but that's would also be the big point. If it was or wasn't, should worth a lot have the candidate in right place.
*I think that test was proved the candidates/my skills to *
* Analyze a foreign system
* Use a minimal of information to find errors and bug
* Under time stress and without someone help you, code assumed corrections
* Different levels of knowledge;
** sql db and stored procedures,
** use of dll in project,
** asp.net technique,
** layered architecture
** problem-oriented aspect
But also the more obvious things like handle the developer environment, find and understand Db Server Management tool. Surely there are candidates that looks really nice on paper but, in practice, could stuck on such tasks.