Behavior like that suggests that the IDE is miscounting characters in the file. There are several "file oddities" that the developers might not have anticipated and that could cause a character index to be miscalculated.
Check your file for unusual line endings. For example, a line missing a carriage return will appear correctly in the editor, but can cause incorrect line numbers elsewhere in the program.
Non-breaking spaces are another kind of character that will appear normally, and maybe even parse correctly, but occupies multiple bytes when represented in UTF-8 when the "normal" version of the character (a regular space) only takes up one. You will sometimes acquire such characters if you copy code from a Web browser.
Incorrect byte-order marks for UTF-8 or UTF-16 source files might also cause problems.
Often, opening your source file in Notepad or a hex editor will help you identify these problems.