You can certainly try

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"You can certainly try" is a phrase often used by Dungeon Masters, especially Matt Mercer, to indicate that, while there is nothing in the rules preventing a player from attempting a given course of action, that player doesn't necessarily have any chance of success, and the consequences of trying could be very, very bad.

Alternately, can be used of an option that is clearly a "trap build" of some kind, such as playing a race with Level Adjustment in Dungeons & Dragons 3e.

Examples of "You can certainly try"

• Playing any kind of Dragon or half-Dragon in pre-4e D&D. (Dragonborn seem to have filled this niche nicely.)
• Playing a monster in a setting not set up for it.
• Using a lasgun on a defensive shield.