“Good luck” is often a very positive phrase, but not always.  In some situations the English phrase can be understood cynically or negatively as in “good luck – you’ll need it,” in which case the phrase might inspire worry, not confidence.

A billboard advertisement for Uzbekistan Airways in Tashkent International Airport inspires a huge lack of confidence with the phrase “good luck.”  The billboard features a jet buried almost completely in the clouds with only the tail visible and below it in large letters the phrase, “good luck.”  This was probably intended to express well wishes like the phrases “have a nice flight” or “bon voyage.”  However, that English phrase is not so commonly used to express well wishes for a journey in some English-speaking countries.  The airplane buried ominously in clouds and large snowflakes further supports the unintended negative message, “good luck – you’ll need it with this airline.”

Source: Airliners.net