one-horse-wagon: Doesn't Moser et al. (2018) "An Empirical Analysis of Traceability in the Monero Blockchain" do what you're asking for, more or less?

Actually, we discover some (many) horrors in the Monero blockchain thanks to heavy

(and probably long) investigations, and only lately.

Maybe one doable thing would be to have volunteers realizing from time to time

some specialy designed txs from various exchanges/wallets and,

knowing themselves those txs, take notice of suspicious behaviour (eg bad formed ring).

Suspicious txs could then be collected somewhere (could eg be a github repo or thread).

If enough weird txs concern often the same exchange, this could be useful info for Monero users.

It's probably not easy to have such a collective watch organized,

but I wonder if it could be an interesting idea, or not ?

It's a shame that the only entities with that visibility of spend patterns are the big exchanges, and I doubt they will share their findings

I don't think we need to watch big patterns to notice that something is wrong.

eg, if there is some web-wallet which systematically forges bad rings, this behaviour could be catched just by making a tx sample from time to time.

s/eg, if there is some web-wallet which systematically forges bad rings, this behaviour could be catched just by making a tx sample from time to time./eg, if there is some web-wallet which **systematically** forges bad rings, this behaviour could be catched just by making a tx sample from time to time and dissecting this tx./

* something is going wrong.

Or "somehow" the decoy selection algorithm could be enforced. At one point enforcing at the consensus level was considered, but ArticMine suggested that it be done at the node re-broadcast level instead, which could be a nice compromise. See:

Halver[m]: it was gate dot io and was first noticed in 2018