What the cap applies to, and for how long
The bonus max bet rule applies for as long as a bonus has open wagering requirements, and against all spins.
It is written either as a fixed sum, or as a proportion of the bonus, the two forms are not interchangeable. A proportion is the cleaner plan, because it is written once and stays with the bonus even if the bonus is broken by cashback or by a deposit added in. A fixed sum has to be written to take both those cases, and if it is written as a sum before the cashback or deposit, it is too low. This can make it unclear what the max stake is.
Why nothing stops you at the spin
The casino software does not have to reject a stake that goes over the max bet threshold. The player can place stakes that exceed the max bet without immediate notification. This returns the player to a point where their deposit, their bonuses, and their winnings are not theirs to control. The only charge the player knows at the moment is the total sum that the bonus and the deposit together paid in.
The bonus terms' max bet is not rejected at the spin: it is enforced at the payout review, It means that, while the player is spinning, the terms are accepted by the reality of play, they cannot opt out along the way by raising a query or getting a warning. Play can continue for days before any issue is detected at withdrawal. A player can end up playing a bonus for hours or for days, then at the withdrawal time discover it voided, and that they took a stake that was over the max. They are given the choice of playing on without the bonus, or they can take the deposit out, but leave the winnings behind.
There are real-world practicalities to these examples. A stake can be entered without a player's knowledge, because of the autoplay, or because of a stake accidentally left in place from an earlier session. Options like feature buys or double-ups can create large stakes. Each one of these cases is a gap between the act and its consequence, and in each one the max stake is neither checked at the spin, nor enforced then.
Three ways to break it by accident
The cap can be breached by accident, in three ways.
A feature buy can suddenly become a huge stake, and one that the player did not anticipate. A feature buy gives the player the chance to enter a spins bonus or a free spins round as a one-off payment, so that they enter it without having to hit the right spins to trigger it.
A gamble option, or a double-up, also opens up a way for a player to take a big, one-off stake. When this is in place, and a win is fed through the game's auto-cashout, that win is made into the stake for the next play, and the max bet is breached.
A stake can be shared over more than one play using autoplay — and that is a way to break the maximum stake rule. So can leaving a stake in place from an earlier session. Both ways are common causes of a breach, and between them they can set the player up for days of play where the stakes are high, and going over the max.
Where it is written down
The bonus max stake is written in the bonus terms, and never in the promotion banner or in a general terms and conditions section. The terms in force at the moment of the promotion are the binding ones, and they may be updated at any time.
Voiding the bonus is not the same as voiding the deposit. The terms for each bonus date set out how the deposit funding the bonus is won or lost, if a stake goes over the max bet. The terms specify whether voiding affects just the bonus or also the deposit.
