Seen in IE8 and IE7 since Nov 9 2012 in vector at least, IE9 and above seems OK:
in English Wikipedia
type any text into the search box
do not select or click any suggested text that appears below the search box
click the magnifying glass icon
suggested text disappears
no search occurs, user is left at original page
workaround: click the suggested text that appears below the search box, and search will complete
several tickets in OTRS refer to this
Version: unspecified
Severity: major
OTRS tickets: 2012110910007595 2012111010007351 2012111110002273
2012111210003378 2012111210002851
https://de.wikipedia.org/w/index.php?title=Wikipedia:Fragen_zur_Wikipedia&oldid=110472467#Suche
(In reply to comment #1)
OTRS tickets: 2012110910007595 2012111010007351 2012111110002273
2012111210003378 2012111210002851
Also, 2012111210000648
Gerrit changeset 33287: Apparently IE7 (and IE8 in compatability mode) use 'type="button"' as the default for <button> tags rather than 'type="submit"'. So clicking the button doesn't submit.
Oh look, someone already submitted almost the same thing as gerrit change 30819. Merged.