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#softwareGripe search features that don't give any hint as to what the rules are.
If I just enter a string, will it match everything that includes that string, or that starts with it, or only items that match it exactly? Do wildcards work, or will they be taken literally, and cause my search to never find anything?
Lookinatchu, #Caja... 
@woozle also love software which implicitly drops words that are "too common" even if they're being used in an exact phrase, and has no way to override that behavior
phpBB is especially awful at that one