Filling in a space between the letters of a word
It is permissible to erase an extra letter in the middle of a parsha. But if the erasure would leave a gap in the middle of a word so that it looks like two words, the parsha is pasul. In some cases it is possible to solve the problem by lengthening the top/base of a preceding letter (beis, chaf, reish, etc.) or thickening the wall/leg of the following letter. Care must be taken not to alter the form of the letter in doing so.
(סעיף כג וס"ק קיב)