English words that are frequently used in the German language can easily become false friends when used with the same meaning in English. A classic example is Handy, the widely used term for mobile phone (or cell phone), which will certainly confuse native speakers a lot. While very few advanced speakers make the mistake of trying to use handy in English, a surprisingly common false friend is the word beamer (Beamer in German) used in reference to a data projector. Just pay attention to conversations at seminar talks or conferences! Amusingly, beamer is a colloquial name for BMW cars and motorbikes,* rendering the phrase
My Office is too small for a beamer,
as overheard in a recent lunch time conversation, truly comical. Finally, why not avoid this mistake by using the proper German word, namely Projektor? (Apparently, enough German speakers do not understand the word projector to justify the inclusion of beamer in English documents, see here.)
Even though most German speakers may understand what you mean when you say beamer instead of projector, native English speakers will typically be completely lost.
* Evidence for the use of the word beamer for BMW cars and motorbikes can be found in the Oxford English Dictionary, see here. An alternative spelling is beemer. Moreover, as explained here, BMW enthusiasts use beamer/beemer for cars and beamer for motorbikes. (Thanks to Eric for the link.)