This document is an overview of common mistakes made by authors on SoL, and by other writers of erotica. There are several other guides posted on this site to help you become a better writer, and this document will not attempt to duplicate the same information. Instead, it is meant as a guide to help both new and existing writers avoid some of the most common mistakes and make their writing more readable.
Poison Pen's Writer's Guide includes a section on sexual accuracy. Two mistakes in particular are very common:
The hymen is not inside the vagina! It is part of the vulva, near the entrance to the vagina. This mistake shows up so often that storiesonline has posted a page with more information (link is not safe for work). In addition, there is no concept of being "technically" or "physically" not a virgin due to the absence of the hymen; virginity is related only to having sexual intercourse. If you are going to write about the hymen, or the clitoris, or the g-spot, make sure you know where they are located.
Withdrawal is not a very effective method of birth control (if your characters believe it is, that's a different matter). While most of the sperm is released during ejaculation, some are released with the pre-ejaculate lubrication produced for intercourse. Even if a man withdraws before ejaculating, a small number of sperm are already in the vagina, so the possibility of pregnancy exists.
This is not a common mistake, but it's an embarrassing one for those authors that make it, so here it is: A woman does not urinate through her vagina, but through her urethra (which is below the clitoris and above the vagina). For a picture of the various parts, see the link above to the page Lazeez posted about the hymen.
A common mistake made on SoL is using the wrong word, either a word that sounds the same, or a word that is spelled similarly to the one you are looking for. Use your spell-checker to catch typos, but do not depend on it to select the correct word for you. Some common examples:
lose (opposite of 'find') vs. loose (opposite of 'tight')
your (showing ownership) vs. you're (contraction of 'you are')
woman (singular) vs. women (plural)
quite (completely, actually, or rather) vs. quiet (opposite of 'loud')
they're (they are) vs. their (ownership) vs. there (location and all other meanings)
two (the number) vs. too (also/excessively) vs. to (all other meanings)
defiantly, when what you really meant was 'definitely'
whose (to designate ownership) vs. who's (the contraction of who is, or who has)
payed should be 'paid' in almost all cases (the exception is a nautical term, as in, 'he payed out the rope')
aureole (the sun's corona) vs. areola (the dark skin around the nipple)
then (moment in time - mostly) vs. than (contrast with)
Dragged (past tense for Drag) vs. Drug (Medicine or narcotic)
Thought (past tense for Think) vs. Thunk (a noise)
Discrete (distinct) vs. Discreet (prudent)
By far the most common mistake made by amateur writers is the incorrect use of commas and semi-colons. Using punctuation correctly makes a big difference in how readable your story is. When commas and semi-colons are used in the wrong places or a sentence is incomplete, it throws the reader off and can hurt the flow of the story; it can even change the meaning of what you have written. If it happens too often, some readers will give up on your story for that reason alone.
The other guides on this site discuss how to use punctuation correctly, and plenty of in-depth punctuation guides can be found through Google. At the time of this writing, the first hit on Google is to this site. In particular, pay attention to the pages titled 'The Comma' and 'The Semicolon.'
A frequent error made with commas on SoL is neglecting to include a comma after a parenthetical element (a section of text that interrupts a sentence to offer additional explanatory information).
Incorrect: Lisa, the farmer's daughter knocked on the bedroom door.
Correct: Lisa, the farmer's daughter, knocked on the bedroom door.
Another frequently-made error is to add commas in places where they do not belong. See the section titled "Superfluous Commas" on the comma page in the punctuation guide linked above.
Semicolons should only be used in two cases:
The most common case is to separate independent clauses (clauses which could each be a complete sentence on their own). In this case, you would use the semicolon in place of a period, and you would not capitalize the beginning of the second clause. If the clauses could not be complete sentences on their own, use a comma instead. See the semicolon and comma pages in the punctuation guide linked above for further information.
Incorrect: Lisa, the farmer's daughter, knocked on the bedroom door, she was dressed in a pale negligee.
Correct: Lisa, the farmer's daughter, knocked on the bedroom door; she was dressed in a pale negligee.
Less often, a semicolon is used to separate items in a list, provided one or more of those list items uses commas internally.
Punctuation in and around quoted dialogue.
Incorrect: "I'm going to school." She said.
Correct: "I'm going to school," she said.
Apostrophes, and Its vs. It's
Apostrophes should be only be used to show possession, or for contractions. They should never be used to show that a word is plural.
A special case, or at least a common mistake, is the word it's. It's is used only to show the contraction, 'it is.' Use its, without the apostrophe, to show possession, since its is a possessive pronoun (like 'hers' or 'yours'). Possessive pronouns do not use apostrophes.
Oxford University Press on proper uses of the apostrophe:
It's is not, it isn't ain't, and it's it's, not its, if you mean it is. If you don't, it's its. Then too, it's hers. It isn't her's. It isn't our's either. It's ours, and likewise yours and theirs.
-- Oxford University Press, Edpress News