Can a thermometer be wrong?

My son was acting tired but it was 830 anyway I took his temperature just to be sure and it said 100.0 under his arm. He didn't feel that warm to me. *he went to sleep* but anyways I felt my daughter and she is not warm AT ALL * she's not cold she just don't feel temperature warm* it said 99.9 under her arm. So I checked mine and it was the same 99.9. Do you think my thermometer is wrong? Is that possible?