Sometimes I mix my hiragana and katakana characters within a word.

[AExAVF]

I just received the results of my assignment, and noticed an error in one of my answers.  We were assigned to write the katakana of the word purezento/present (プレゼント) but I only realized that I wrote the hiragana letter of pu, and as a result the answer is (ぷレゼント).  Mixing the hiragana and katakana together within a phrase or sentence is acceptable, but not within a word.

Has anyone committed this particular mistake while writing the hiragana/katakana of the words?

[petrushka]

I've never personally had a problem mixing katakana with hiragana since they look so different to me, but katakana itself used to be a problem for me because of the similarities between シ with ツ and ン and ソ and sometimes even ノ.

And then, there are the kanji that look like simple katakana or hiragana words likes 力 (chikara) and カ (katakana ka). I just can't spot the difference and it's even worse if it was handwritten. I don't know the technique to figure it out but I'm still looking and learning, so perhaps I'll figure it out soon when I get used to reading context easier than just relying on how similar it looks.