1- Focusing on food; to the extent that people begin to worry about eating more then actually fasting. This also goes along with spending tons of money on Iftaars even though a person does not need to eat that much food.
2- Making Suhoor way before Fajr. Some people eat Suhoor a few hours after Taraweeh or Isha Salah, this is wrong. It should be eaten closer to the time of Fajr.
3- People don't make Niyyah (intentions) to fast for Ramadaan. This is something in the heart and does not need to be verbal.
According to the Hanabili school it's considered one worship and only needs to be done once, at the beginning of Ramadhan unless there is a break in the fasting.
According to Malik, Shafii, Abu Hanifah and some Hanablis the intention must be renewed nightly.
4- If you find out late that Ramadaan started, you should stop eating and fast for that day, making that day up after Ramadaan/Eid ends.
5- Many people don't think you pray Taraweeh on the first night of Ramadaan. They believe you pray it after the first day you actually fast. They forget that the Islamic calender runs on the moon, maghrib is the start of the new day.
6- Many people believe if you eat or drink on accident this breaks your fast. This is false, if you do this on accident then you continue fasting and do not need to make up the day.
7- Some people take the opinion that if they see someone eating or drinking they should not remind the person that he/she is fasting. According to Sheikh Bin Baz may Allah have mercy on him, this is incorrect and it is an order from Allaah for us to ordain the good and forbid the evil. Thus we tell the person, because we are forbidding the evil this way.
8- Many sisters believe they cannot use Hennah while fasting. This is incorrect, they are allowed to use it during Ramadaan.
9- Some people believe when you are cooking you cannot taste the food to see if it has the right spices/flavors. This is false, and allowed in Islam as long as the person cooking is not eating the food. Rather they can taste it to see if it needs salt, or more spices.
10- Many people think you cannot use a Miswak or toothbrush during Ramadaan. This is false, for the Prophet peace be upon him used to use a miswak during Ramadaan. Also you can use toothpaste; the reasoning by the scholars is that the Miswak has flavor, thus toothpaste is okay to use (if you are not eating it).
11- Some people make the Fajr adhaan early. They do this so people will stop eating before Fajr and not invalidate their fast. This is wrong and something we should not do.
12- Some people make the Maghrib adhaan late. They do this so people will start eating late, just in case Maghrib has not come in yet. This too is wrong and we should not do this.
13- Many many people believe you cannot have intercourse with your spouse during the whole month of Ramadaan. This is false, you cannot do this only during the times when you are fasting. Between Maghrib and Fajr it is permissible to do.
14- Many women believe that if their period has just ended and they did not make ghusl, they cannot fast that day (considering their period ended at night, and they went to bed without Ghusl, waking up without having a chance to make it). This is incorrect, if a women has not made Ghusl she can still fast.
15- Many men believe that if he has had intercourse with his wife and did not make ghusl (similar to the above) then he cannot fast the next morning. This is also incorrect, for he can fast even if he has not made Ghusl.
16- Some people pray Duhr and Asr prayers together during Ramadaan. (mainly in Arab countries) This is incorrect and should be avoided.
17- Some people believe you cannot eat until the Muadhin is done calling the Maghrib Adhaan. This is incorrect, as soon as he starts a person can break their fast.
18- Many people don't take advantage of making du'aa before they break their fast. Allah accepts a fasting persosn du'aa. Many ualama recommend it right before Maghrib
19- Many people make the mistake of spending the later part of Ramadaan preparing for Eid, neglecting Ramadaan. This is incorrect and these people lose the concept of what Ramadaan is about.
20- Many parents do not let their children fast during Ramadaan (young children). This is something counter productive to a child. By allowing him to fast he will grow up to know he must do this act.
21- Many people think Ramadaan is just about not eating and forget about controlling their tempers and watching what they say. In actuality we are supposed to control our tempers and mouths even more during Ramadaan.
22- People often waste their time during Ramadaan. They go to sleep during the day and get nothing done. We should be taking advantage of this blessed month by doing extra Ibaadat.
23- Some people don't go on trips or travel during Ramadaan. They think they have to break their fast when traveling. This is actually optional, if you want to break your fast while traveling you can (with making it up later), and if you don't you can continue fasting.
24- Many people who are able don't make Itikaaf at the masjid. We should take advantage of our good health and spend lots of time at the Masjid, expecially the last 10 days of Ramadaan.
25- Some people believe they cannot cut their hair or nails during Ramadaan. This is also false.
26- Some people say you cannot swallow your spit during Ramadaan. This too is false. However you cannot swallow mucus that has entered your mouth.
27- Some people say you cannot use scented oils or perfumes during Ramadaan. This too is false.
28- Some people believe bleeding breaks the fast. This is not true.
29- Some people believe if you throw up on accident it breaks your fast. This is not true, however if you do it intentionally it does.
30- Some people think you cannot put water in your nose and mouth during wudu in Ramadaan. This too is incorrect.
31- One should not fast a day or two before Ramadaan, unless it is a day on which one is in the habit of fasting (i.e. voluntary fasting that coincides with that day).