Are computers infallible? (Not sure if this is the right place to ask this question.)

Let's say I get a computer to calculate 1+1. It should give 2, obviously. Will it always give 2? Is there any possible combination of circumstances that could cause a computer to, after finishing its calculations, give a number other than 2?

To illustrate what I'm thinking of, for humans the answer is "no" because if they're (e.g.) sufficiently drunk, they might get it wrong.
An algorithm may be infallible. Any physical computer can make an error e.g. if it is hit by a stray cosmic ray, a bolt of lightning or a large axe.