Test find(...) >= 0 (or < 0), never == -1. In H21 find() on an array returns -(len+1) for a missing value (-2 for a one-element array), not -1.
set*attrib() writes are deferred until the wrangle finishes. point()/prim()/ detail() in the same wrangle read the INPUT. Keep multi-pass state in local arrays and write once at the end, or split the passes into chained wrangles.
An empty group pattern selects ALL elements (expandpointgroup(0, ""), SOP group fields). Guard parameter-driven patterns with if (pat != "").
nearpoints() includes the point itself. Skip pt == @ptnum when counting or averaging neighbours, or the count is one too high and the average is pulled towards the point.
chs()/chf() read stale values when spare parms are added after the wrangle has cooked (e.g. build_network then create_spare_parameters). Create the spare parms first, or re-set the snippet / force a recook after adding them, then re-read the result.
A wrangle built from code fails silently from the caller's side. A compile error (e.g. bare P.y = 1; - "Reference to undefined variable", always write @P) shows only in node.errors() or cook(force=True), so force-cook and check errors on every node you build before saving or reporting.

How to check your result

Read the result back from Houdini before reporting it: point/prim counts, attribute value histograms (not one sample), and two or three values worked out by hand. A plausible-looking viewport is not a check.
After adding or changing parms on a node that already cooked, recook and read again before diagnosing anything (trap 5).
When a wrangle "does nothing": write each intermediate value to a debug attribute and read them back; compare against the input to spot trap 2.
Never report a cause you have not tested. If the first fix did not change the measured result, the diagnosis is still open - say so. In the 2026-11 eval, 5 of 9 runs told the user an invented cause for what was really trap 5.

Reference

reference/wrangle_traps.md - each trap with the incident, fix snippet and the read-back that proves it.
SideFX docs through the Houdini MCP: search_help / get_help_page (fxhoudinimcp, port 8100). Build wrangles with build_network and check them with verify_network.

2.0.0 (2026-11-08) - Cut to a traps list after a no-skill baseline showed no lift (ClaudeCode/evals/houdini-vex-v2, 21 voluntary runs never opened the skill; forcing it cost +33%). General VEX tutorial removed; buggy nearpoints snippet replaced by trap 4; added stale chs() after spare parms (observation 442). Dropped the ternary string-type trap: it did not reproduce in H20.5.613 or H21.0.596 (hython re-test 2026-11-08).