For example, 丫 is a character while ㄚ is a bopomofo symbol. I find it very difficult to tell the difference (especially if they are hand-written), unless from the context.
Other examples:
幺andㄠ; 又andㄡ,一andㄧ;彳andㄔ;ㄙand厶, etc.
They are so similar that I failed to see how one could distinguish them visually, especially in handwritting.