When I've been in a game for a while (noticed most frequently on realms, probably because I play realms most), I'll be typing either to a person or typing a command, and one letter when hit will either not enter into the game, or will highlight the whole word or whole section of what was typed. Trying to type further with it highlighted will write over what was already typed. The only way for me to correct the problem, has been to close out of minecraft completely, and to restart the program.

Just now, I was trying to type an /effect command, and the "c" wouldn't work until I restarted the game.