The Location: At the house of a Pharisee named Simon, who had been a leper (Mt. 26:6; Mk. 14:3; Lk. 7:36, 40). John's Gospel seems to suggest it was at the home of Lazarus, Martha, and Mary (Jn. 12:1-2).
The Woman: Mary is named as the woman in John's Gospel (12:3), but the other three simply say "a woman" (Mt. 26:7; Mk. 14:3; Lk. 7:37).
The Anointing: Two of the Gospels mention that she anointed His head (Mt. 26:7; Mk. 14:3), while the other two say she anointed His feet (Lk. 7:38; Jn. 12:3). All four describe the precious material as being an "ointment" (Mt. 26:7; Mk. 14:3; Lk. 7:37; Jn. 12:3). Three mention that it was in a box (Mt. 26:7; Mk. 14:3; Lk. 7:37), while John simply says it was "a pound" (12:3). Its value was said to be "300 pence" (Mk. 14:5; Jn. 12:5), Matthew says the ointment "could have been sold for much" (Mt. 26:9), and Luke does not mention its value.
The Message: Three state that her devotion was for His burial (Mt. 26:12; Mk. 14:8; Jn. 12:7), while Luke seems to be saying her action was more out of gratitude for she knew Jesus would forgive her sins (7:40-50).
Since all four Gospels are Scripture, and therefore true, I believe each is a "witness" to a single event. A Pharisee named Simon, a former leper from Bethany, had as his servant, Martha. Lazarus was a guest, and Mary, their sister, was the repentant sinner. Mary came uninvited, and showed her devotion to Jesus by anointed both His head and His feet with a very expensive ointment normally used for preparing a body for burial. That the poor will always be with us, that Judas was a thief, and that Simon needed to learn to be more hospitable and not to judge, was the purpose of the Lord's visit.
All Scripture is truth; therefore it is impossible for it to contradict itself!