Worst TAing/Teaching experience.

[humanino]

$ds=cos(x)\rightarrow \frac{ds}{s}=co(x) \rightarrow d=co(x)$

I think it's a rather comic situation. While in class, a student made quite a decent calculation on the board and came up with the result
$$\frac{\sin\theta}{\cos\theta}$$
and thought he was done. I loved my math teacher, I swear he was wearing the same clothes every single day, I'm sure he had 5 or 10 identical red shirts. Anyway, obviously he was expecting a tangent, so he insisted on the last step, but the poor student did not remember it. So the teacher took the chalk, and wrote
$$\frac{\sin\theta}{\cos\theta}=\frac{in}{co}$$
For a few moments there was despair in the student's eyes. Few of us burst into laughs, and the teacher apologized.

 

[humanino]

Oh there was also this other time when the physics teacher was drawing a solenoid, a current flow, the direction of the field, erased this and that, then draws the expected trajectory, and ends up with a huge smiley on the black board. As soon as she realized why we were laughing, she erased it promptly.

 

[Moonbear]

There are some quantities that must always be positive. One cannot have a negative number of apples, for example.

Hmmph! Of course you can! You make it a positive number by planting apple seeds. (Afterall, if the spreadsheet can have negative dollars, surely one can have negative apples for the ones owed to someone else.)