Energy
Players set the phone clock forward to refill their hearts for free
Any heart timer that believes the phone's clock can be fooled in seconds. Here is a way to keep honest time between visits to the server, notice when the clock was moved, and what to do after a restart, without punishing people who travel.
Undo gives back the move but not the booster, and the next tiles come out different
A player undoes a move and finds the booster still spent, or the new pieces that drop in are not the same ones. Both break trust. Here is what an undo must restore, what it must not, and when it has to refuse.
A free "unlimited lives" gift should pause the heart clock, not waste it
Players love a half hour of unlimited lives, until they notice it ended with fewer hearts than they started with, or that a second gift reset the first. Here is how to make the gift add up fairly and stay honest with the refill.
Energy comes back faster on a fast computer than on a slow phone
If a game adds a point whenever a counter passes a limit, the result depends on how many pictures per second the device draws. The same game then plays differently everywhere. Here is the way to make every device agree, and where the error hides.
A rest bonus for time away either never runs out or vanishes in one fight
Rewarding players who come back after a break with a temporary boost is kind, but easy to get wrong. The boost can pay out per action instead of per point earned, grow without limit, or be applied twice. Here is a bank that behaves.
Sprinting flickers on and off at the empty point because the bar refills a hair at a time
When stamina hits zero and any tiny refill lets the player sprint again, the character stutters and the bar twitches. Here is how to make running out mean something, and a recovery that feels like catching your breath.