HELP I can't login to my account

Is it just me or is 1 in every ~200 clients just plain nuts?

Just about every day I must get 5 emails stating "Hey, can't login right now, what's up?" or "Your login form is broken".

This happens every day, when I have changed nothing. Meanwhile a few thousand clients seem to be able to login just fine. That, or they're incredibly shy.

Is this a common occurrence among password requiring websites? Or is my simple php cookie implementation perhaps too simple and people are indeed having real problems?