One feature in OS X that I use on a regular basis is the system-wide spell checker, which is available in any application built to include this function (an easy thing to do with Apple's developer tools). In OS X 10.5 and prior, misspelled words were marked by a dotted red underline, but in Snow Leopard the new autocorrection feature is sometimes frustrating.
In Snow Leopard, Apple changed the default behavior of the spell checker to automatically correct misspelled words. While this is useful in many instances, the there are some instances where this feature can be frustrating.
While one would expect common mistakes (i.e., the frequent "teh" typo) to be corrected, the system may automatically correct a word if there is only one found possibility in the spelling dictionary. This coupled with the lack of an "undo" feature for the word correction makes it so when a word is not recognized by the system and subsequently changed, you cannot easily change it back to the intended spelling. Undoing the system's change may result in the system changing it right back as it was before the change, but will place your cursor on the word so when you continue typing the misspelled word changes again.
As a result, if you enter a word like "Knowledgebase" (technically "Knowledge Base" but commonly used together in one word) the system will quickly change it to "Knowledgeable." Pressing Command-Z to undo the change reverts it to "Knowledgebase," but when you continue typing the word immediately changes again to "Knowledgeable." The only way to keep "Knowledgebase" is to type a few more words and then go back and change the word back to what you want.
There is no way to fix this behavior if it happens, so the only workaround is to turn off the automatic correction in the system. This is available in the "Edit" menu under the "Spelling and Grammar" submenu, as one of the three behavior options for managing spelling and grammar in documents. Changing these options will affect applications globally, so you only have to set them in one application. Keep in mind that you will only be able to set the automatic spelling correction option if the active application both supports this function and if there is an active text field.
With automatic correction disabled, you can still have the system check spelling as you type and underline questionable words, but it will not change them for you. Unfortunately there is no way to specify which words will be automatically corrected or have just a known subset of words be corrected automatically, so with the feature off you will also have to manually deal with words like "teh." Hopefully Apple will enhance this feature in future OS releases, because it is definitely one of the more regularly used ones in OS X.