Someone told me that we must be careful when doing certain wazifas, what does that mean? is there any danger?

Each wazifah contain a specific effect to the reader. It is not advisable for instance to keep on the air-conditioned even if it is already cold inside. Or you may take it as a medication. One should not overdose and follow the prescription of the doctor. So one need to consult an alim to see what you are reading are continuously beneficient. Some wazifas are not masnoon and should give place to masnoon ones.