Histograms

Treating the vertical axis as frequency rather than frequency density.

Some students simply add the heights; you must use area = frequency.

Frequency density is $\dfrac{\text{frequency}}{\text{class width}}$, so plotting the raw frequency as the bar height is wrong whenever class widths differ. A class of $45$ values over a width of $5$ has density $9$, not $45$.

Class width is the upper boundary minus the lower boundary, and the intervals often differ: $0 \le t < 20$ has width $20$ but $20 \le t < 30$ has width $10$. Using one width for every bar, or misreading the boundaries, throws off every frequency you recover.

The vertical scale is frequency density, and its gridlines rarely go up in ones. Read the actual value on the axis before multiplying by the width — assuming each square is $1$ unit is a frequent, avoidable slip.