yes, in few cases some calculators do have errors. Most of the time is the scientific functions that have errors, answers with decimal places, the answers are slightly off, basic functions have errors too. Try comparing with 2 calculators.
Yes, errors can occur due to "floating point accuracy". Since calculators operate in binary (i.e. use 1s or 0s to do maths) representations of fractions can become innacurate. But on most calculators, you have to go quite a long way before you can notice an error!
almost all calculators get minus numbers to the power of a an even number worng. try -2 squared and the calculator will give the answer -4, when it obviously should be 4. dont know why exactly it does this but anyway it has some advanced purpose but it means it gets these basic things wrong.