Most definitely. The calibration tends to drift during the operation. In a store I worked in it drifted so much that handling merchandise near the front door started setting it off. Eventually it requires a field tech to come out to recalibrate.

The pads that disable the tags tend to drift, too. Sometimes to the point where they'll disable all the tags around the cash register and sometimes to where they wont disable any at all. Which is particularly obnoxious as every person walking out of the store ends up setting the alarm off.

The vast majority of false-positives I noticed came from merchandise purchased elsewhere that hadn't had its tag deactivated. Makeup seems to be a popular culprit.

This was all with a CheckPoint system (which uses RFID), though, so I'm not sure about magnetic systems.