Probably one of the most common mistakes is forgetting to put the parentheses on some functions.
For example, if I were going to graph #y = 5^(2x)# as stated in a problem, some students may put in calculator 5^2x. However, the calculator reads that it is #5^2x# and not as given. So it is important to put parentheses in and write 5^(2x).
For logistic functions, one error can involve using natural log vs. log incorrectly, like:
#y = ln(2x)#, which is #e^y = 2x#; versus
#y=log(2x)#, which is for #10^y = 2x#.
Exponent conversions to logistic functions may be tricky as well. If I were to graph #2^(y) =x# as a y-function of x, it would be:
#log_2(x) = y# or #log(x)/log(2)=y# in calculator.
These are few of the mistakes most people tend to make. The best way to prevent this is to practice and to be careful on inputting the values so that those functions are good to graph.
If there are more mistakes that I have not mentioned, feel free to add some more.