The most common errors in undergraduate mathematics
<i>By the way, write your plus sign (+) and lower-case letter Tee (t) so that they don't look identical!</i><p>From my experience, I'm one of the very few people who write manually in a serifed font; in particular, I write 1, l, I, and | very distinctly and use a slashed zero so as to distinguish it from the letter O. I wish more people would do this since it is still sometimes necessary to communicate on paper, and trying to interpret handwritten notes with ambiguous writing can be quite annoying.<p><i>Unquestioning faith in calculators.</i><p>The same can be said for computers too - except the problem is often worse as computers are far more complex to the point that almost no one understands them fully, and we are encouraged to accept this.