Germany was 'not amused' when news surfaced in the 90's that the Turkish army was using former East German military equipment (including APCs) against Kurds in its civil war.
The reason was that Germany largely prohibits arms exports into crisis regions (with few cabinet-level waivers). We still manage to export a huge load of arms every year, of course.
The U.S. could 'potentially' become embarrassed by seeing Egyptian M1 Abrams tanks or their predecessors, the M60 Patton tank (both of U.S. origin) rolling over demonstrators in Cairo very soon.
The M1 and the M60 are the most numerous and afaik also the best-maintained main battle tanks of the Egyptian army, after all.
This could produce a lot of ugly pictures and could raise additional eyebrows over the Western policy towards Arab autocrats.
The good thing about it is that this event is rather unlikely, since many army 4x4 APC crews have already shown solidarity with the demonstrators, yelling "We are all brothers!". The Egyptian army's loyalty to the Mubarak regime seems to be questionable at the junior leadership levels right now.
edit: I saw already a press photo of an abandoned M60 (in the streets of either Suez or Cairo).
edit2: I saw a first photo of a column of M1s in the streets, but apparently again not opposing the crowd as most likely intended by Mubarak.