So if you are sat on the ground and the temperature rises by 25 degrees (for the sake of example) and you do not change your altimeter setting, your altimeter will give you a HIGHER reading i.e. your plane will be LOWER than your altimeter reading.
If the airfield's elevation is zero and sea-level pressure is set in the altimeter sub-scale, my understanding is that the altimeter will indicate true altitude regardless of the temperature. Temperature errors occur when an aircraft is above mean sea-level.
If an aircraft is above msl and the temperature is higher than standard then the altimeter will under-read (indicate an altitude which is lower than its true altitude). This is essentially a safe error. If however the temperature is lower than standard the altimeter will over-read. This is potentially hazardous for obvious reasons.