Amphiboly
http://datanation.com/fallacies/amphib.htm

Definition:
An amphiboly occurs when the construction of a sentence
allows it to have two different meanings.

Examples:
 (i) Last night I shot a burglar in my pyjamas.

(ii) The Oracle of Delphi told Croseus that if he pursued the
     war he would destroy a mighty kingdom. (What the Oracle
     did not mention was that the kingdom he destroyed would
     be his own. Adapted from Heroditus, The Histories.)

(iii) Save soap and waste paper. (From Copi, p. 115)

Proof:
Identify the ambiguous phrase and show the two possible
interpretations.

References
Copi and Cohen: 114