With both Mist Dragon and Crackdown Construct on the battlefield, I activate Mist Dragon's giving it flying and making it lose flying, if I do this 1,000 times, would this give the Construct +1,000/+1,000 until end of turn?
Assuming you control Mist Dragon and Crackdown Construct:
You can activate Mist Dragon's first or second ability as often as you like (C.R. 118.5, 118.5a, 602.2, 602.2b, 601.2h); noting that neither ability is a mana ability (C.R. 605.1a), doing so will make Crackdown Construct's ability trigger that many times (C.R. 603.2). If the latter ability resolves 1000 times this turn, Crackdown Construct will get +1000/+1000 until end of turn.
EDIT (May 17): Add rule citations.
EDIT (Nov. 22, 2019): Edited, including because some rules were renumbered with Core Set 2020.
You're also not required to alternate between Mist Dragon's first two abilities. You can activate the Mist Dragon ability to give it flying when it already has flying.
BUWGRChilds PlayGRWUB
BUWGR Highlander GRWUB
UBSquee's Shapeshifting PetBU
BW Multiplayer Control WB
RG Changeling GR
UR Mana FlareRU
UMerfolkU
B MBMC B
You can activate Mist Dragon's first or second ability as often as you like (C.R. 118.5, 118.5a, 602.2, 602.2b, 601.2h); noting that neither ability is a mana ability (C.R. 605.1a), doing so will make Crackdown Construct's ability trigger that many times (C.R. 603.2). If the latter ability resolves 1000 times this turn, Crackdown Construct will get +1000/+1000 until end of turn.
EDIT (May 17): Add rule citations.
EDIT (Nov. 22, 2019): Edited, including because some rules were renumbered with Core Set 2020.
Two Score, Minus Two or: A Stargate Tail
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