Are there any expressions to describe being compliant in particular occasions?

For a single lapse, there's the very common expression [have / show] a moment of weakness:

a moment of weakness – a short time during which someone makes a bad decision or does something bad that they would not normally do

• He kept asking me to help him, and in a moment of weakness I said yes.

[Cambridge Dictionary]

Though it's a hypernym, it's very often used when someone complies tamely when they should say no. It's also often obvious humerous semi-hyperbole, as in the following example:

Since I started working with Dan Clarke, he’s been nagging me to do one his .NET Oxford talks on the subject .... Anyone who knows Dan will know that he doesn’t take no for an answer – so in a moment of weakness, I agreed to do it....

[Dave England_.Net Oxford]

You might use the phrase 'drop the ball', which implies a misjudgement or lapse in a particular circumstance or instance, and also (I think) implies that the lapse is uncharacteristic*.

Examples

1. Derek is a solid chap. It was most unlike him to drop the ball when forgot to take a face mask to the shops.

2. Oi, Dave, you've dropped the ball there, my son. Arsenal-Spurs was never going to be a boring, nil-nil draw.

(* I say this because, if a person were always to be 'dropping the ball', it would get to the point that they were considered generally 'useless'.)

docious-Ready to accept instruction or direction, obedient, subservient

deferential-Marked by or showing careful attention to set forms and etiquette Acting or tending to diminish or avoid conflict or hostility Adjective