You have to ask yourself what was the last thing you added to FS. If you down loaded a airplane that used strange textures than saved that flight as the default FS just might keep the textures even though you are no longer in that aircraft. But I have to admit that it would be a little strange to do that. But I have seen FS do some odd things. You may have to remove what ever the last thing you added to FS.

Nine out of ten times something you added will cause strange problems that were not there before. Beware of repaints as some designers get the textures wrong.
If you got a error message warning about a duplicate file when adding some thing and you chose to over write the orignal file you are stuck with it.
BACK UP BACK UP before you do anything to your orignal FS