A few I can think of...
Philip Cousin told me when he was on Wheel, they told him and the others not to look at the wheel upon spinning it. They didn't want anyone trying to finesse the wheel spin to land exactly where they wanted it, and if they suspected you looked at the wheel, they'd make you re-spin.
Randy Amasia told me and a group of game show fans that on Whew!, they were discouraged from saying "I don't know" as a response to a blooper if you didn't know... either make a wild guess or say nothing, not that it was a rule, just something they discouraged.
Not really a rule, but Frank Wayne later had an aversion to potential contestants saying "I don't believe it!" on TPIR. He would discourage the producers from picking contestants that uttered that phrase in line.