No index is perfect. Because of that genealogists should always be aware of limitations of indexes that they are using. Genealogists with any amount of research time under their belt should be aware of incorrect transcriptions on the part of the indexer and incorrect renderings of the name on the part of the clerk.
But there are other things to consider and remember.
Does the index include every name in the record? If not, what names are included? Many local indexes to land records only include the name of the first grantor and the first grantee. Some index all grantors and grantees, but those indexes tend to be the exception to the rule. In some locations, land records are in one consolidated series of indexes, in others each volume of records is indexed separately.
Court records generally index the first plaintiff and the first defendant listed in a case, but there are exceptions to those items as well. Some court indexes give the file number to a the “case file” and include references to the court case in various journals and order books. Some do not. In some cases volumes are indexed separately instead of being compiled into a consolidated sort of index.
Modern census indexes are usually every name indexes.
Indexes of digital images of newspapers (and some city directories) are computer generated indexes that, while helpful, can easily misread type that was only partially legible or blurred on the microfilm copy that was used to create the digital image.
Not every record series is completely extant. There can be gaps.
One way to get some practice in using an index is to find a random item (or two, or three) in the original record and see how it appears in the index.
For any index, I usually try and ask myself “what potential limitations are there to this index?” If I don’t of any, I try and find out what they are. Every index has them. If I am unaware of them, it impacts my ability to use the index effectively.