Why does text get selected when I type too quickly?

When I try to type quickly sometimes text gets selected (sometimes a word, phrase, or even whole line) and then when I subsequently (before I can react to the selecting) hit a key it overwrites (deleting) whatever is selected and in apps that don't have an undo I end up losing thing that just got overwritten - incredibly frustrating!!! Am I doing something wrong? Is this a setting I can turn off? Is it a bug?

btw, using this text field is proving to be pretty frustrating too (unable to register touch selections without docking keyboard and then going back into entry mode, also stops typing in certain situations) - beginning to wonder if these are bugs left to be worked out...?