Steps to reproduce

For example, search this issue (https://jira.atlassian.com/browse/JRA-5379) in issue navigator:

There is an underscore in the summary (OS_CURRENTSTEP).

So, try this JQL query :

project = JRA AND summary ~ OS_ AND priority =
Minor AND resolution = Unresolved ORDER BY summary ASC, key DESC

It "works" : it returns the issue 5379

(among other issues it should not like "OS X" which has a space but no underscore)

Try second JQL query :

project = JRA AND summary ~ OS_C AND priority = Minor
AND resolution = Unresolved ORDER BY summary ASC, key DESC 

(just add a C after the OS_)

It does not work anymore...

So, the bug seems reproductible on Atlassian's JIRA.