A social-signal system stores rows from many channels in one posts table, tagging each row's channel in a single column ("reddit-sub-name", "bluesky:search:<brand>", "pinterest:search", "google:trending"). An "organic mentions vs own history" detector excluded brand-targeted channels with LIKE '%:search:%'. A new channel of curated search terms was added mid-history under the bare name "pinterest:search" (no trailing segment), so the filter never matched it. Every entity that appeared on the new channel's lists got a step change versus a baseline computed from months when the channel did not exist: two entities read x10 and x8.5 "above their usual" on ONE real post each plus ~26 list rows. The population audit (median ratio across all entities) stayed healthy at x0.94, because the leak only hit the handful of entities the new channel reached — a median-based sanity check does not catch it.

An agreement detector flags an entity when at least two independent streams are above the entity's own median in the same window. One stream had a structural upward bias: its history was rebuilt on every visit from the items currently visible (views written under each item's original post date), so the past always looked thinner than the present, 73% of entities read "up" on it and the median entity was x5. Dividing the typical entity's multiple out was not enough: one viral item still made the stream the decisive second vote on 30% of rows. Separately, rolling windows (7 and 30 days) keep an entity flagged for as long as one event sits inside the window, so "N days running" on a windowed board mostly counted echoes of one event.

A daily anomaly report persisted each board's rows to a history table, and the operator then asked for weekly and monthly reports. Reusing the daily section builders for the period pages had two traps: those builders wrote their rows to the history table as "today's" record, so a period page rebuilt with a different window silently overwrote the daily record; and one board that lists every tracked entity every day (a universe list, not a ranking) made every entity a "seen every day" regular in the roll-up. Early in the history, "seen 3 days" also read as strong when only 3 days had ever been recorded.

A daily report had "week" and "month" views of an anomaly rule (an entity fires when a stream is above its own median) but no "day" view, and adding one naively either flooded the board with one-day counting noise or duplicated the rendering code with hard-coded period words ("this week" printed on the month board). Rebuilding the page for a same-day rerun also re-ran every network collector, doubling request loads on rate-limited or fragile accounts.

You want to find companies currently valued below what large strategic corporate investors paid for their stakes. Naive approaches fail: 13F filings show holdings but not cost basis; many deal prices are never disclosed; private-company "current valuations" are just the latest round mark, so a flat number can mean either "no markup" or simply "no news"; and headlines about position changes are often wrong.

A multi-source attention system had a dozen boards that all ranked entities by how much louder than usual they were. Reviewing the three cases that actually worked for the operator showed the opposite shape: small entities with no broad attention, whose own community was slowly tightening (more posts from the same people, more scarcity language). None of them ever reached the top of a board, so the system could not put the right kind of candidate in front of the operator. A second failure: the universe builder added communities that merely existed (newest post six years old, one private since spring) and every board read their silence as a quiet week.

Building a watch-list of small companies from a financial data feed and pairing each with its community (a subreddit) produced confident false matches: a one-word company name resolved to a community about a Norse god, an Australian suburb, an English city, a video game, or a generic word ("Designer"). Company names from the feed also carried corporate suffixes ("Holdings, Inc.", "Industries", "Group") and were truncated at ~31 characters with share-class fragments ("..., Inc. -", "International, In"), so aliases nobody types were written and could never match a post.

A daily sampler visits N items from a list using a saved cursor, so the whole list is refreshed over several days. Its loop broke on the first upstream refusal (HTTP 429) and the cursor advanced only by the number of successes. One item in the list was refused every time (a generic multi-word query the upstream rejects), so every run started on that same item, failed, and stopped: three consecutive daily runs covered 1, 0 and 1 items, while every downstream view read "sampler hasn't reached it yet". Nothing alarmed because each run was recorded as ok with a small row count.

A multi-source "agreement" board (a brand appears when several independent streams are above their own baseline) was dominated by one stream: a video platform sampler whose history is reconstructed from the videos still visible today, so the past is always thinner than the present and every entity looks like it is rising (82% of judged brands fired, median ratio x5). Rows carried by that stream looked like strong multi-signal agreement. Two related artifacts: a series that starts with empty weeks (collection reached the entity late) counted 0>=0 as consecutive rising steps and printed "25 weeks climbing, x26" on flat data; and a rotating sampler reaching an entity for the first time was reported as the entity "appearing" with an infinite ratio.

A pipeline health check judged every data source on the same "newest row older than 2-3 days = stale" rule. Two sources legitimately write slower: one serves weekly aggregated points (its newest day is 7-13 days old on any morning) and one is validated ~7 days late by the provider. Both alarmed every single morning while healthy. Three of eight daily problems were false, which trains the operator to stop reading the list and miss the real failures (a scraper starved on four days that month).