The table below presents presents two contradictory sets of verses. Both verse sets talk about offerings for sin. The underlined words highlight the contradiction. One verse set says bring a guilt offering if you sinned without being sure while the other verse says bring a sin offering if you are sure you sinned. Which is it? If you weren't sure whether you sinned and then became sure: Do you bring a guilt offering or sin offering. Rashi simply resolves this using the 2 stages method: When a person thinks that perhaps he has sinned inadvertently - for example, forbidden and permissable food are before him, he ate one, and doesn't know which one he ate - he brings the guilt offering for doubtful sin. If he afterwards finds out that he definitely sinned - for example, he is able to verify that he ate the prohibited food - he then brings a sin offering.

Summary	Verse / Source	Text of verse / Source
If you don't know (for certain) if you have sinned, then bring a guilt offering.	Lv05-17:18b	And if a soul sins, and commits any of these things which are forbidden to be done by the commandments of the Lord; though he knew it not, yet he is guilty, and shall bear his iniquity. #####
If you know for certain (that you have sinned) then bring a sin offering	Lv04-27:28	And if any one of the common people sins through ignorance, when he does something against any of the commandments of the Lord concerning things which ought not to be done, and is guilty; Or if his sin, which he has sinned, comes to his knowledge; then he shall bring his offering, a kid of the goats, a female without blemish, for his sin which he has sinned.
Resolution:	2 Stages:	When a person thinks that perhaps he has sinned inadvertently - for example, forbidden and permissable food are before him, he ate one, and doesn't know which one he ate - he brings the guilt offering for doubtful sin. If he afterwards finds out that he definitely sinned - for example, he is able to verify that he ate the prohibited food - he then brings a sin offering.

Advanced Rashi: Several supplementary points should be made about this Rashi.
• From the repetition he doesn't know in Lv05-17,Lv05-18 Rashi infers that ignorance is an essential prerequisite for the guilt offering - in other words, if the ignorance is cancelled, then the expiation of the guilt offering is cancelled and he must bring the sin offering. Here Rashi uses the formatting method where this rule could have also been presented.
• We could have also approached this Rashi using the alignment method: Here we would contrast the verses if he did not know...he brings a guilt offering with if he knew...he brings a sin offering.
• The use of the contradiction method here is a bit arbitrary. The two verses if he doesn't know then he brings a guilt offering while if he does know he brings a sin offering don't really contradict each other; rather they complement each other. There is only one aspect of contradiction - namely the case where he initially didn't know and then later found out. For this reason I have classified the resolution of this contradiction as the two stage method.