Corrections of clerical or grammatical errors are usually allowed, insofar as it is evident that an error has occurred and what the correction should be.
Some examples of allowable corrections:
the replacement of "respectfully" by "respectively" in a claim. (T 34/03
the addition of a plural "s" to the word "particle" as the corresponding verb "have" was in the plural form, and the application as originally filed described a particle size distribution. Since particle size distributions can be defined only for a plurality of particles, the correction was held allowable. (T 108/04
On the other hand, an applicant cannot rely on:
a mere count of the number of instances of the relevant words in the application as originally filed for obtaining the replacement of one word by another word, for instance the substitution of "included" for "excluded", if it is not clear that an error has occurred and not possible to ascertain that nothing other than "included" was intended by the drafter. (T 337/88
usual practice or industry standards for measuring concentrations of compounds in the relevant technical field, if the application as originally filed merely refers to "%", without clarification as to whether by weight or volume, and the description contains no clear guidance as to whether "%" refers to concentration by % by weight or % by volume or to something different. (T 48/02
common general knowledge in the absence of further evidence, such as an encyclopaedia or basic textbook, to argue for instance that the skilled person would have immediately recognised that an ASTM standard with a six-digit number did not exist before the priority date of a patent. (T 881/02