When in Special:ListUsers you can click on contribs to see what the user has contributed, but this gives the following exception:
(replaced the username with <username> below.
Exception encountered, of type "InvalidArgumentException"
[f16de82ee9227a9173d25e69] /index.php/Special:Contributions/<username> InvalidArgumentException from line 246 of /u01/mediawiki/includes/Message.php: $key must be a string or an array
#0 /u01/mediawiki/includes/GlobalFunctions.php(1391): Message->__construct(NULL, array)
#1 /u01/mediawiki/includes/changes/ChangesList.php(203): wfMessage(NULL)
#2 /u01/mediawiki/includes/specials/pagers/ContribsPager.php(451): ChangesList::flag(string)
#3 /u01/mediawiki/includes/pager/IndexPager.php(442): ContribsPager->formatRow(stdClass)
#4 /u01/mediawiki/includes/specials/SpecialContributions.php(204): IndexPager->getBody()
#5 /u01/mediawiki/includes/specialpage/SpecialPage.php(479): SpecialContributions->execute(string)
#6 /u01/mediawiki/includes/specialpage/SpecialPageFactory.php(576): SpecialPage->run(string)
#7 /u01/mediawiki/includes/MediaWiki.php(282): SpecialPageFactory::executePath(Title, RequestContext)
#8 /u01/mediawiki/includes/MediaWiki.php(745): MediaWiki->performRequest()
#9 /u01/mediawiki/includes/MediaWiki.php(519): MediaWiki->main()
#10 /u01/mediawiki/index.php(43): MediaWiki->run()
Running MediaWiki 1.27.1 with php 5.6.17.
When I wanted to check contribs for user Admin when logged in as user Admin it showed the contributions so I thought it could be a security-issue, so I tried with my username and was also only able to see the contribs of Admin.
So I did another test with another user which only contained letters and then it doesn't throw an error. It seems only to result in an error when the username is in the format AB012345, so first a few letters followed by numbers.