Calling doubtful impurity "tumat hatehom / impurity of the depths": Zevachim 23a
Telling a person to repeat his action in order to determine whether contact with impurity could have occurred: Niddah 5b, 58a
Dealing with a case where the current situation indicates that the entity is impure, as in where the entity it touched is currently dead, but there was an established status of purity before the event took place: Eruvin 35b-36a
Considering a minority percentage of cases to be non-existent: Niddah 18b
Cases of doubt, where the impurity was caused by a human being: Niddah 5b
Cases of doubt of Vessels in Jerusalem: Pesachim 19b
Where there is a real and substantial doubt: Tos. Pesachim 9b #1
Where the Case of Doubt is with an item which cannot be questioned to determine its status: Pesachim 19b
Case of Doubt for Impurity which is a result of human action is automatically considered to be a Case of Doubt in something which Can be questioned: Pesachim 19b-20a
Case of Doubt whether one touched Impurity in a Private Area: Pesachim 10a
Case of Doubt whether he entered the Area of the Impurity in the first place: Pesachim 10a
If an axe was lost in a house, or was found in a location other than the one in which it was placed, it is [not] assumed that an Impure stranger entered and made the objects in the house Impure: Pesachim 10b
Case of Doubt in Public Property
A case of doubt in Public Property is presumed pure: Pesachim 19b; Niddah 3a, 4b, 5b, 18a