Continuity mistake: In a full length view of the two forts, Sam's fort has a flag reading "They" and Bugs' fort has a flag with "We" on it. When we see Sam tunnelling underground, the words on the flags are missing.
Continuity mistake: When we first see the "They" flag on Yosemite Sam's fort, it is completely fine. When Sam says "Ahoy, there!" there is now a hole in the top right hand corner.