Unexpected entry in a field! How did this happen?

Unexpected entry in a field! How did this happen?

In one of the forms on my website, I have a <select> field like this:
<select name="doorno" required>
<option></option>
<option>01</option>
<option>02</option>
<option>03</option>
<option>04</option>
<option>05</option>
<option>06</option>
<option>07</option>
<option>08</option>
<option>09</option>
<option>10</option>
<option>11</option>
<option>12</option>
<option>13</option>
<option>14</option>
<option>15</option>
<option>16</option>
<option>17</option>
<option>18</option>
<option>19</option>
<option>20</option>
<option>21</option>
<option>22</option>
<option>23</option>
<option>24</option>
</select></p>
The user should be able to select numbers between 01 - 24 - right?
But in the table where these entries are stored, I see an entry like:
01,05
How could this have happened? When I try to type into the field I am not
able to it - I can only choose one of the options in the list?
Any idea how the user could have managed to enter an option like the one
above?