Confused by squiggly lines
You can find that the numbers "6" and "8" are clearly different. However readers can be confused by squiggly lines behind captchas. The two numbers could confuse the reads by one of those squiggles. The captcha can also make a ‘c’look like an ‘o’, an ‘o’ look like an ‘a’ or other letters by the placement and font.
Upper or lowercase
Captchas could be case-sensitive sometimes though generally they do not care if the letters are upper or lowercase. You have to enter characters in upper and lowercase when the captcha is showing both upper and lowercase letters. You may ignore the case when you happen to see all the letters are shown in uppercase. Only in special you should type letters in uppercase once the entry is rejected.
The case is much sensitive
The captcha will give more confused letters if they are case-sensitive. You can find the letter ‘o’ or ‘c’ are showing the same whether they are of uppercase of lowercase.
What if the captcha just cannot see clearly
You may even find it is difficult or impossible to reduce the vision of the capthcas. So some captchas will appear as an audio version to get around the problem. You will feel it is much easier to hear the codes than to type them by the nice option.
Reloading code is a good way
You can try to reload the captcha when you can not sure if you can pass the current code. You can do it anytime as the system allows. To get an easier-to-read captcha you could reload the images by click the button again and again.